[iOS WK2] Interactive elements of developer.apple.com are broken

[iOS WK2] Interactive elements of developer.apple.com are broken
https://bugs.webkit.org/show_bug.cgi?id=143692
Source/WebCore:

rdar://problem/19320087

Reviewed by Sam Weinig.

When a composited RenderLayer had nodes in the scrolling tree by virtue of
both position and overflow:scroll, and one of those reasons disappeared,
we'd fail to remove the corresponding node from the scrolling tree. This
could leave an overflow:scroll element behaving as if it were position:fixed.

Fix by having RenderLayerCompositor::updateScrollCoordinationForThisFrame()
detach the layer on a per-role basis.

Test: platform/ios-simulator-wk2/scrolling/remove-scrolling-role.html

* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::detachFromScrollingCoordinatorForRole):
* rendering/RenderLayerBacking.h:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::detachScrollCoordinatedLayerForRole):
(WebCore::RenderLayerCompositor::detachScrollCoordinatedLayer): Just moved.
(WebCore::RenderLayerCompositor::updateScrollCoordinatedLayer): Call detachScrollCoordinatedLayerForRole()
if the layer doesn't have the relevant scrolling reasons.
* rendering/RenderLayerCompositor.h:

LayoutTests:

Reviewed by Sam Weinig.

* platform/ios-simulator-wk2/scrolling/remove-scrolling-role-expected.txt: Added.
* platform/ios-simulator-wk2/scrolling/remove-scrolling-role.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@182799 268f45cc-cd09-0410-ab3c-d52691b4dbfc