Reviewed by Sam Weinig.

        Fix scrollbar rendering crash when rendered into a layer
        https://bugs.webkit.org/show_bug.cgi?id=54494

        The scroller delegate was failing to return a layer in a case
        where the scroller was still rendering into a GraphicsLayer, because
        even though WebCore had switched out of compositing mode, the
        DrawingAreaImpl was still rendering into its nonCompositedContent
        layer. This happens when the m_exitCompositingTimer timer hasn't fired
        yet.

        The fix is to track whether we're drawing into a layer via the
        GraphicsContext, rather than asking about compositing layers.

        * platform/graphics/GraphicsContext.h:
        * platform/graphics/cg/GraphicsContextCG.cpp:
        (WebCore::GraphicsContext::setIsCALayerContext):
        (WebCore::GraphicsContext::isCALayerContext):
        Getter and setter for the bit that says if we're rendering into a CALayer.

        * platform/graphics/cg/GraphicsContextPlatformPrivateCG.h:
        (WebCore::GraphicsContextPlatformPrivate::GraphicsContextPlatformPrivate):
        Store a bool for whether this context targets a layer.

        * platform/graphics/mac/WebLayer.mm:
        (drawLayerContents): Call setIsCALayerContext(true).

        * platform/mac/ScrollAnimatorMac.h:
        (WebCore::ScrollAnimatorMac::setIsDrawingIntoLayer):
        (WebCore::ScrollAnimatorMac::isDrawingIntoLayer):
        New methods to get and set a flag that says when we're
        drawing into a layer, so that the delegate can get at this state.

        * platform/mac/ScrollAnimatorMac.mm:
        (-[ScrollbarPainterDelegate layer]): Use isDrawingIntoLayer()
        (WebCore::ScrollAnimatorMac::ScrollAnimatorMac): Init the flag.

        * platform/mac/ScrollbarThemeMac.mm:
        (WebCore::ScrollbarThemeMac::unregisterScrollbar): Whitespace.
        (WebCore::ScrollbarThemeMac::paint): Push the "isDrawingIntoLayer"
        state down onto ScrollAnimatorMac, getting it from the GraphicsContext.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@78612 268f45cc-cd09-0410-ab3c-d52691b4dbfc