Skip clearing the canvas before painting the contents of opaque layers

cc/resources/content_layer_updater.h

Index: cc/resources/content_layer_updater.h
diff --git a/cc/resources/content_layer_updater.h b/cc/resources/content_layer_updater.h
index 6c8dee3bba70ca39e4dbddeae01589acb757e228..28cc384bce3f11d835b006a363f12246f9876f5a 100644
--- a/cc/resources/content_layer_updater.h
+++ b/cc/resources/content_layer_updater.h
@@ -22,6 +22,7 @@ class RenderingStatsInstrumentation;
 class CC_EXPORT ContentLayerUpdater : public LayerUpdater {
  public:
   void set_rendering_stats_instrumentation(RenderingStatsInstrumentation* rsi);
+  virtual void SetOpaque(bool) OVERRIDE;
 
  protected:
   ContentLayerUpdater(scoped_ptr<LayerPainter> painter,
@@ -36,6 +37,8 @@ class CC_EXPORT ContentLayerUpdater : public LayerUpdater {
                      gfx::Rect* resulting_opaque_rect);
   gfx::Rect content_rect() const { return content_rect_; }
 
+  bool layer_is_opaque() const { return layer_is_opaque_; }
+
   RenderingStatsInstrumentation* rendering_stats_instrumentation_;
   int layer_id_;
 
@@ -43,6 +46,10 @@ class CC_EXPORT ContentLayerUpdater : public LayerUpdater {
   gfx::Rect content_rect_;
   scoped_ptr<LayerPainter> painter_;
 
+ protected:
+  // True when it is known that all output pixels will be opaque.
+  bool layer_is_opaque_;
+
   DISALLOW_COPY_AND_ASSIGN(ContentLayerUpdater);
 };