Re-land r158052 after removing ASSERT_NOT_REACHED that caused crash.

Source/core/rendering/RenderLayerBacking.h

Index: Source/core/rendering/RenderLayerBacking.h
diff --git a/Source/core/rendering/RenderLayerBacking.h b/Source/core/rendering/RenderLayerBacking.h
index 6e388527935ebc964226463326cdc4174acf50c6..0a17874cec6d3c9a59fa3ab3c2cebe18df0422f5 100644
--- a/Source/core/rendering/RenderLayerBacking.h
+++ b/Source/core/rendering/RenderLayerBacking.h
@@ -37,6 +37,7 @@ namespace WebCore {
 
 class KeyframeList;
 class RenderLayerCompositor;
+class WebAnimationProvider;
 
 enum CompositingLayerType {
     NormalCompositingLayer, // non-tiled layer with backing store
@@ -244,9 +245,6 @@ private:
 
     void doPaintTask(GraphicsLayerPaintInfo&, GraphicsContext*, const IntRect& clip);
 
-    static CSSPropertyID graphicsLayerToCSSProperty(AnimatedPropertyID);
-    static AnimatedPropertyID cssToGraphicsLayerProperty(CSSPropertyID);
-
     RenderLayer* m_owningLayer;
 
     // The hierarchy of layers that is maintained by the RenderLayerBacking looks like this:
@@ -316,6 +314,8 @@ private:
     OwnPtr<GraphicsLayer> m_layerForVerticalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
 
+    OwnPtr<WebAnimationProvider> m_animationProvider;
+
     IntRect m_compositedBounds;
 
     bool m_artificiallyInflatedBounds; // bounds had to be made non-zero to make transform-origin work