Reduce compositing update in Silk's toggle_drawer by 20%

Source/core/rendering/compositing/GraphicsLayerUpdater.h

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 23 matching lines @@
 class GraphicsLayer;
 class RenderLayer;
 class RenderPart;
 class RenderView;
 
 class GraphicsLayerUpdater {
 public:
     explicit GraphicsLayerUpdater(RenderView&);
     ~GraphicsLayerUpdater();
 
-    void updateRecursive(RenderLayer&);
-    void rebuildTree(RenderLayer&, Vector<GraphicsLayer*>& childLayersOfEnclosingLayer, int depth);
+    enum UpdateType {
+        DoNotForceUpdate,
+        ForceUpdate,
+    };
+
+    void updateRecursive(RenderLayer&, UpdateType);
+    void rebuildTree(RenderLayer&, UpdateType, Vector<GraphicsLayer*>& childLayersOfEnclosingLayer, int depth);
 
 private:
-    void update(RenderLayer&);
+    UpdateType update(RenderLayer&, UpdateType);
 
     RenderView& m_renderView;
 
     // Used for gathering UMA data about the effect on memory usage of promoting all layers
     // that have a webkit-transition on opacity or transform and intersect the viewport.
     double m_pixelsWithoutPromotingAllTransitions;
     double m_pixelsAddedByPromotingAllTransitions;
 };
 
 } // namespace WebCore
 
 #endif // GraphicsLayerUpdater_h