cc: Throttle tile priority updates to once a frame.

cc/layer_impl.h

Index: cc/layer_impl.h
diff --git a/cc/layer_impl.h b/cc/layer_impl.h
index c05fb5539017b73a4b69a4cc12b6e115325f61da..b02db83489562965d4f146ddbb4da6098fb24902 100644
--- a/cc/layer_impl.h
+++ b/cc/layer_impl.h
@@ -107,6 +107,8 @@ public:
     virtual RenderPass::Id firstContributingRenderPassId() const;
     virtual RenderPass::Id nextContributingRenderPassId(RenderPass::Id) const;
 
+    virtual void updateTilePriorities() { }
+
     virtual ScrollbarLayerImpl* toScrollbarLayer();
 
     // Returns true if this layer has content to draw.
@@ -278,7 +280,6 @@ public:
 
     virtual Region visibleContentOpaqueRegion() const;
 
-    virtual void didUpdateTransforms() { }
     virtual void didBecomeActive();
 
     // Indicates that the surface previously used to render this layer