Reviewed by Sam Weinig.

        GraphicsLayers in subframes can get sync'd multiple times
        https://bugs.webkit.org/show_bug.cgi?id=52489

        Some cleanup that will work towards fixing this bug.

        Tested by existing iframe compositing tests.

        * WebCore.exp.in: syncCompositingStateRecursive()
        was renamed to syncCompositingStateIncludingSubframes().

        * page/FrameView.h:
        * page/FrameView.cpp:
        (WebCore::FrameView::syncCompositingStateForThisFrame): Some
        code cleanup: do all the word we need to do for this frame,
        including the needsLayout() check.
        (WebCore::FrameView::syncCompositingStateIncludingSubframes):
        This is no longer recursive; instead, it iterates over descendant
        frames via the frame tree, calling syncCompositingStateForThisFrame()
        on each Frame's view.

        * rendering/RenderLayerCompositor.h:
        (WebCore::RenderLayerCompositor::isFlushingLayers): Getter for the flag.
        * rendering/RenderLayerCompositor.cpp:
        (WebCore::RenderLayerCompositor::RenderLayerCompositor):
        (WebCore::RenderLayerCompositor::flushPendingLayerChanges): Maintain
        a flag to say if we're flushing, which allows us to assert on re-entrant flushes.
        (WebCore::RenderLayerCompositor::enclosingCompositorFlushingLayers):
        Add the ability to get the rootmost compositor that is in the middle
        of a flush.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76196 268f45cc-cd09-0410-ab3c-d52691b4dbfc