Don't create render passes for transparent images.

cc/layer.h

 // Copyright 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_LAYER_H_
 #define CC_LAYER_H_
 
 #include <public/WebFilterOperations.h>
 #include <string>
 #include <vector>
@@ skipping 249 matching lines @@
     bool automaticallyComputeRasterScale() { return m_automaticallyComputeRasterScale; }
     void setAutomaticallyComputeRasterScale(bool);
 
     void forceAutomaticRasterScaleToBeRecomputed();
 
     // When true, the layer's contents are not scaled by the current page scale factor.
     // setBoundsContainPageScale recursively sets the value on all child layers.
     void setBoundsContainPageScale(bool);
     bool boundsContainPageScale() const { return m_boundsContainPageScale; }
 
-    // Returns true if any of the layer's descendants has content to draw.
-    bool descendantDrawsContent();
+    // Returns 0 if none of the layer's descendants has content to draw,
+    // 1 if exactly one descendant has content to draw, or a number >1
+    // (but necessary the exact number of descendants) otherwise.
+    int descendantsDrawContent();
 
     LayerTreeHost* layerTreeHost() const { return m_layerTreeHost; }
 
     // Set the priority of all desired textures in this layer.
     virtual void setTexturePriorities(const PriorityCalculator&) { }
 
     bool addAnimation(scoped_ptr<ActiveAnimation>);
     void pauseAnimation(int animationId, double timeOffset);
     void removeAnimation(int animationId);
 
@@ skipping 138 matching lines @@
 
     WebKit::WebAnimationDelegate* m_layerAnimationDelegate;
     WebKit::WebLayerScrollClient* m_layerScrollClient;
 };
 
 void sortLayers(std::vector<scoped_refptr<Layer> >::iterator, std::vector<scoped_refptr<Layer> >::iterator, void*);
 
 }  // namespace cc
 
 #endif  // CC_LAYER_H_