Plumb CompositingReason from Blink to compositor.

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 29127e0125eb527a825f733f302cea116d077c0f..15bf17168f1c462031bcedfd07d0171cfecd27df 100644
--- a/Source/core/rendering/RenderLayerBacking.h
+++ b/Source/core/rendering/RenderLayerBacking.h
@@ -180,7 +180,7 @@ private:
     void createPrimaryGraphicsLayer();
     void destroyGraphicsLayers();
     
-    PassOwnPtr<GraphicsLayer> createGraphicsLayer(const String&);
+    PassOwnPtr<GraphicsLayer> createGraphicsLayer(const String& name, CompositingReasons);
 
     RenderLayerModelObject* renderer() const { return m_owningLayer->renderer(); }
     RenderLayerCompositor* compositor() const { return m_owningLayer->compositor(); }
@@ -233,6 +233,8 @@ private:
     void updateBackgroundColor(bool isSimpleContainer);
     void updateContentsRect(bool isSimpleContainer);
 
+    void updateCompositingReasons();
+
     bool hasVisibleNonCompositingDescendantLayers() const;
 
     bool shouldClipCompositedBounds() const;