https://bugs.webkit.org/show_bug.cgi?id=121714
<rdar://problem/15042478>

Reviewed by Beth Dakin.

After r155660, we don't do any additional layouts when scrollbars are
added or removed. That caused us to not enter the code that creates or
destroys composting layers for scrollbars in RenderLayerCompositor, resulting
in missing scrollbars.

Fix by having ScrollView::updateScrollbars() call addedOrRemovedScrollbar()
when a scrollbar was added or removed, which is overridden in FrameView
to call through to RenderLayerCompositor::frameViewDidAddOrRemoveScrollbars().

Not testable since scrollbar layers aren't contained in layer tree dumps
(to reduce diffs between platforms).

* page/FrameView.cpp:
(WebCore::FrameView::addedOrRemovedScrollbar): New function to tell the compositor
that scrollbars changed.
* page/FrameView.h:
* platform/ScrollView.cpp:
(WebCore::ScrollView::setHasHorizontalScrollbar): Change the meaning of the return
value to say if the scrollbar was added or removed. Add an out param that indicates
whether the scrollbar change affects content size (not true for overlay scrollbars).
(WebCore::ScrollView::setHasVerticalScrollbar): Ditto.
(WebCore::ScrollView::updateScrollbars): Keep track of whether scrollbars were
added or removed, and call addedOrRemovedScrollbar() if true.
* platform/ScrollView.h:
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::frameViewDidAddOrRemoveScrollbars): Call updateOverflowControlsLayers().
(WebCore::RenderLayerCompositor::updateOverflowControlsLayers):
* rendering/RenderLayerCompositor.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156201 268f45cc-cd09-0410-ab3c-d52691b4dbfc