Merge 98060 - Hidden composited iframes cause infinite loop

Merge 98060 - Hidden composited iframes cause infinite loop
https://bugs.webkit.org/show_bug.cgi?id=52655

Source/WebCore:

Reviewed by Darin Adler.

visibility:hidden is problematic for compositing, because it causes
RenderLayers to be removed from the z-order layer tree. This confuses
RenderLayerCompositor in several ways; it never sees these layers
when traversing the tree as it computes compositing requirements, or
rebuilds the layer tree.

This is a particular problem with composited iframes. When an iframe
becomes composited, scheduleSetNeedsStyleRecalc() is called on that
iframe's ownerElement in the parent document. If this happens inside
Document::updateStyleForAllDocuments(), we get into an infinite loop
because notifyIFramesOfCompositingChange() queues up style update as we
bounce in and out of compositing mode, so documentsThatNeedStyleRecalc
never empties out.

This is an initial, conservative fix that doesn't attempt to fix all
the issues with visibility. It changes RenderLayerCompositor to count
the number of compositing RenderLayers, and to not leave compositing
mode if there are any (even if they are hidden, so not hit while
traversing the z-order tree). This avoids the infinite loop.

Test: compositing/visibility/hidden-iframe.html

* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::ensureBacking):
(WebCore::RenderLayer::clearBacking):
* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::RenderLayerCompositor):
(WebCore::RenderLayerCompositor::hasAnyAdditionalCompositedLayers):
(WebCore::RenderLayerCompositor::updateCompositingLayers):
(WebCore::RenderLayerCompositor::computeCompositingRequirements):
* rendering/RenderLayerCompositor.h:
(WebCore::RenderLayerCompositor::layerBecameComposited):
(WebCore::RenderLayerCompositor::layerBecameNonComposited):

LayoutTests:

Reviewed by Darin Adler.

Test with a visibility:hidden iframe, whose subframe becomes composited.

* compositing/visibility/hidden-iframe-expected.txt: Added.
* compositing/visibility/hidden-iframe.html: Added.

TBR=simon.fraser@apple.com
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=98521