Don't create render passes for transparent images.

cc/layer.cc

 // 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.
 
 #include "cc/layer.h"
 
 #include "cc/active_animation.h"
 #include "cc/animation_events.h"
 #include "cc/layer_animation_controller.h"
 #include "cc/layer_impl.h"
@@ skipping 680 matching lines @@
     setNeedsDisplay();
 }
 
 void Layer::createRenderSurface()
 {
     DCHECK(!m_renderSurface);
     m_renderSurface = make_scoped_ptr(new RenderSurface(this));
     setRenderTarget(this);
 }
 
-bool Layer::descendantDrawsContent()
+int Layer::descendantsDrawContent()
 {
+    int result = 0;
     for (size_t i = 0; i < m_children.size(); ++i) {
-        if (m_children[i]->drawsContent() || m_children[i]->descendantDrawsContent())
-            return true;
+        if (m_children[i]->drawsContent())
+            ++result;
+        result += m_children[i]->descendantsDrawContent();
+        if (result > 1)
+            return result;
     }
-    return false;
+    return result;
 }
 
 int Layer::id() const
 {
     return m_layerId;
 }
 
 float Layer::opacity() const
 {
     return m_opacity;
@@ skipping 116 matching lines @@
 {
     return 0;
 }
 
 void sortLayers(std::vector<scoped_refptr<Layer> >::iterator, std::vector<scoped_refptr<Layer> >::iterator, void*)
 {
     // Currently we don't use z-order to decide what to paint, so there's no need to actually sort Layers.
 }
 
 }  // namespace cc