REGRESSION (r183300): Fixed elements flash when scrolling

REGRESSION (r183300): Fixed elements flash when scrolling
https://bugs.webkit.org/show_bug.cgi?id=144778
rdar://problem/20769741

Reviewed by Dean Jackson.

After r183300 we can detached layer backing store when outside the coverage region.
However, position:fixed layers are moved around by the ScrollingCoordinator behind
GraphicsLayer's back, so we can do layer flushes with stale information about layer
geometry.

To avoid dropping backing store for layers in this situation, prevent backing
store detachment on layers registered with the ScrollingCoordinator as viewport-constrained
layers. Preventing detachment on a layer also prevents detachment on all descendant
layers.

* platform/graphics/GraphicsLayer.h:
(WebCore::GraphicsLayer::setAllowsBackingStoreDetachment):
(WebCore::GraphicsLayer::allowsBackingStoreDetachment):
* platform/graphics/ca/GraphicsLayerCA.cpp:
(WebCore::GraphicsLayerCA::GraphicsLayerCA):
(WebCore::GraphicsLayerCA::setVisibleAndCoverageRects): Set m_intersectsCoverageRect to true
if backing store detachment is prevented.
(WebCore::GraphicsLayerCA::recursiveCommitChanges): Set a bit in the CommitState to
communicate to descendants that detachment is prevented.
* platform/graphics/ca/GraphicsLayerCA.h:
(WebCore::GraphicsLayerCA::CommitState::CommitState): Deleted.
* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::setIsScrollCoordinatedWithViewportConstrainedRole):
* rendering/RenderLayerBacking.h:
(WebCore::RenderLayerBacking::setScrollingNodeIDForRole): If registering with a non-zero
nodeID for the ViewportConstrained role, turn off backing store detachment.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@183970 268f45cc-cd09-0410-ab3c-d52691b4dbfc