Update style/layout when a slot is added or removed

Update style/layout when a slot is added or removed
https://bugs.webkit.org/show_bug.cgi?id=149593

Reviewed by Antti Koivisto.

Source/WebCore:

Fixed the bug by forcing the render tree reconstruction on the shadow host when a slot is inserted or removed.
We should optimize these reconstructions by only triggering them on the affected slot elements in the future.

Also fixed a bug that we were not invalidating the slot assignments when a default slot is introduced dynamically
after the slot assignment algorithm had run.

Test (existing): fast/shadow-dom/shadow-layout-after-slot-changes.html

* dom/ShadowRoot.cpp:
(WebCore::ShadowRoot::addSlotElementByName): Passes in ShadowRoot.
(WebCore::ShadowRoot::removeSlotElementByName): Ditto.
* dom/SlotAssignment.cpp:
(WebCore::SlotAssignment::addSlotElementByName): Call setNeedsStyleRecalc.
(WebCore::SlotAssignment::removeSlotElementByName): Call setNeedsStyleRecalc if the host is still alive since this
function can be called while the host is being destructed in which case shadowRoot.host() would be nullptr.
* dom/SlotAssignment.h:

LayoutTests:

Removed failing test expectations from fast/shadow-dom/shadow-layout-after-slot-changes.html

Also added an explicit test case for when a default slot is introduced dynamically after
calling getDistributedNodes() once, thereby forcing the slot assignments.

* fast/shadow-dom/HTMLSlotElement-interface-expected.txt:
* fast/shadow-dom/HTMLSlotElement-interface.html:
* fast/shadow-dom/shadow-layout-after-slot-changes.html:
* platform/mac/TestExpectations:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@190323 268f45cc-cd09-0410-ab3c-d52691b4dbfc