Direct composite canvas background if possible.

Source/core/platform/graphics/chromium/Canvas2DLayerBridge.cpp

 /*
  * Copyright (C) 2012 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 42 matching lines @@
     , m_next(0)
     , m_prev(0)
     , m_lastImageId(0)
 {
     ASSERT(m_canvas);
     // Used by browser tests to detect the use of a Canvas2DLayerBridge.
     TRACE_EVENT_INSTANT0("test_gpu", "Canvas2DLayerBridgeCreation");
     m_canvas->setNotificationClient(this);
     m_layer = adoptPtr(WebKit::Platform::current()->compositorSupport()->createExternalTextureLayer(this));
     m_layer->setOpaque(opacityMode == Opaque);
+    m_layer->setBlendBackgroundColor(opacityMode != Opaque);
     GraphicsLayer::registerContentsLayer(m_layer->layer());
     m_layer->setRateLimitContext(m_rateLimitingEnabled);
 }
 
 Canvas2DLayerBridge::~Canvas2DLayerBridge()
 {
     GraphicsLayer::unregisterContentsLayer(m_layer->layer());
     Canvas2DLayerManager::get().layerToBeDestroyed(this);
     m_canvas->setNotificationClient(0);
     m_layer->clearTexture();
@@ skipping 197 matching lines @@
 Canvas2DLayerBridge::MailboxInfo::MailboxInfo(const MailboxInfo& other) {
     // This copy constructor should only be used for Vector reallocation
     // Assuming 'other' is to be destroyed, we swap m_image ownership
     // rather than do a refcount dance.
     memcpy(&m_mailbox, &other.m_mailbox, sizeof(m_mailbox));
     m_image.swap(const_cast<SkAutoTUnref<SkImage>*>(&other.m_image));
     m_status = other.m_status;
 }
 
 }