Add slotchange event

Add slotchange event
https://bugs.webkit.org/show_bug.cgi?id=155424
<rdar://problem/24997534>

Reviewed by Antti Koivisto.

Source/WebCore:

Added `slotchange` event as discussed on https://github.com/w3c/webcomponents/issues/288.

While the exact semantics of it could still evolve over time, this patch implements as
an asynchronous event that fires on a slot element whenever its distributed nodes change
(flattened assigned nodes):
http://w3c.github.io/webcomponents/spec/shadow/#dfn-distributed-nodes

Since inserting or removing an element from a shadow host could needs to enqueue this event
on the right slot element, this patch moves the invalidation point of element removals and
insertions from Element::childrenChanged to Element::insertedInto and Element::removedFrom.
Text nodes are still invalidated at Element::childrenChanged for performance reasons
since it could only appear within a default slot element.

Because this more fine-grained invalidation needs to be overridden by HTMLDetailsElement,
we now subclass SlotAssignment in HTMLDetailsElement instead of passing in a std::function.

Test: fast/shadow-dom/slotchange-event.html

* dom/Document.cpp:
(WebCore::Document::enqueueSlotchangeEvent): Added.
* dom/Document.h:
* dom/Element.cpp:
(WebCore::Element::attributeChanged): Call hostChildElementDidChangeSlotAttr.
(WebCore::Element::insertedInto): Call hostChildElementDidChange.
(WebCore::Element::removedFrom): Ditto.
(WebCore::Element::childrenChanged): Don't invalidate the slots on ElementInserted and
ElementRemoved since they're now done in Element::insertedInto and Element::removedFrom.
* dom/Event.cpp:
(WebCore::Event::scoped): slotchange event is scoped.
* dom/EventNames.h: Added eventNames().slotchange.
* dom/ShadowRoot.cpp:
(WebCore::ShadowRoot::invalidateSlotAssignments): Deleted.
(WebCore::ShadowRoot::invalidateDefaultSlotAssignments): Deleted.
* dom/ShadowRoot.h:
(ShadowRoot): Added more fine-grained invalidators, mirroring changes to SlotAssignment.
* dom/SlotAssignment.cpp:
(WebCore::SlotAssignment::SlotAssignment): Removed a variant that takes SlotNameFunction
since HTMLDetailsElement now subclasses SlotAssignment.
(WebCore::SlotAssignment::~SlotAssignment): Added now that the class is virtual.
(WebCore::recursivelyFireSlotChangeEvent): Added.
(WebCore::SlotAssignment::didChangeSlot): Added. Invalidates the style tree only if there
is a corresponding slot element, and fires slotchange event. When the slot element we found
in this shadow tree is assigned to a slot element inside an inner shadow tree, recursively
fire slotchange event on each such inner slots.
(WebCore::SlotAssignment::hostChildElementDidChange): Added. Update the matching slot when
an element is inserted or removed under a shadow host.
(WebCore::SlotAssignment::assignedNodesForSlot): Removed the superfluous early exit to an
release assert since addSlotElementByName should always create a SlotInfo for each element.
(WebCore::SlotAssignment::slotNameForHostChild): Added. This is the equivalent of old
m_slotNameFunction which DetailsSlotAssignment overrides.
(WebCore::SlotAssignment::invalidateDefaultSlot): Deleted.
(WebCore::SlotAssignment::findFirstSlotElement): Added an assertion. slotInfo.element must
be nullptr if elementCount is 0, and elementCount must be 0 if slotInfo.element is nullptr
after calling resolveAllSlotElements, which traverses the entire shadow tree to find all
slot elements.
(WebCore::SlotAssignment::assignSlots):
* dom/SlotAssignment.h: Implemented inline functions of ShadowRoot here to avoid including
SlotAssignment.h in ShadowRoot.h. Not inlining them results in extra function calls for all
builtin elements with shadow root without slot elements, which impacts performance.
(WebCore::ShadowRoot::didRemoveAllChildrenOfShadowHost): Added.
(WebCore::ShadowRoot::didChangeDefaultSlot): Added.
(WebCore::ShadowRoot::hostChildElementDidChange): Added.
(WebCore::ShadowRoot::hostChildElementDidChangeSlotAttribute): Added.
(WebCore::ShadowRoot::innerSlotDidChange):
* html/HTMLDetailsElement.cpp:
(WebCore::DetailsSlotAssignment): Added. Subclasses SlotAssignment to override
hostChildElementDidChange and slotNameForHostChild.
(WebCore::DetailsSlotAssignment::hostChildElementDidChange): Added. We don't check if this
is the first summary element since we don't know the answer when this function is called
inside Element::removedFrom.
(WebCore::DetailsSlotAssignment::slotNameForHostChild): Renamed from slotNameFunction. Also
removed the code to return nullAtom when details element is not open as that messes up new
fine-grained invalidation. Insert/remove the slot element in parseAttribute instead.
(WebCore::HTMLDetailsElement::didAddUserAgentShadowRoot): Don't insert the slot element for
the summary since the details element is not open now.
(WebCore::HTMLDetailsElement::parseAttribute): Remove and insert the slot element for the
summary here instead of changing the behavior of slotNameForHostChild.
* html/HTMLDetailsElement.h:
* html/HTMLSlotElement.cpp:
(WebCore::HTMLSlotElement::enqueueSlotChangeEvent): Added. Enqueues a new slotchange event
if we haven't done so for this element yet.
(WebCore::HTMLSlotElement::dispatchEvent): Added. Clear m_hasEnqueuedSlotChangeEvent when
dispatching a slotchange event so that a subsequent call to enqueueSlotChangeEvent would
enqueue a new event. Note scripts call EventTarget::dispatchEventForBindings instead.
* html/HTMLSlotElement.h:

LayoutTests:

Added a W3C style testharness.js test.

* fast/shadow-dom/ShadowRoot-interface-expected.txt:
* fast/shadow-dom/ShadowRoot-interface.html: Don't import testharness.css from svn.webkit.org.
* fast/shadow-dom/slotchange-event-expected.txt: Added.
* fast/shadow-dom/slotchange-event.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@198115 268f45cc-cd09-0410-ab3c-d52691b4dbfc