[Extra zoom mode] Add plumbing for next and previous focusable element rects

[Extra zoom mode] Add plumbing for next and previous focusable element rects
https://bugs.webkit.org/show_bug.cgi?id=184016
Work towards <rdar://problem/38758727>

Reviewed by Tim Horton.

When building up AssistedNodeInformation, we currently compute the element rect of the current focused element,
as well as flags indicating whether or not there are next or previous focusable elements. For
<rdar://problem/38758727>, we additionally send the rects of the next or previous focusable elements as well.

* Shared/AssistedNodeInformation.cpp:
(WebKit::AssistedNodeInformation::encode const):
(WebKit::AssistedNodeInformation::decode):

IPC support for nextNodeRect and previousNodeRect.

* Shared/AssistedNodeInformation.h:
* WebProcess/WebPage/ios/WebPageIOS.mm:
(WebKit::elementRectInRootViewCoordinates):

Add a helper to compute an element's rect in root view coordinates. We use this to compute the rects of the
current focused element as well as those of the next and previous elements, if any.

(WebKit::WebPage::getAssistedNodeInformation):
(WebKit::hasAssistableElement): Deleted.

Since we need the next or previous focusable element to get its rect, we don't need this helper anymore.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@229982 268f45cc-cd09-0410-ab3c-d52691b4dbfc