WebGL: optimize to draw WebGL on external destination. e.g. 2D Canvas.

Source/platform/graphics/gpu/DrawingBuffer.h

 /*
  * Copyright (c) 2010, 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 228 matching lines @@
     OwnPtr<WebGraphicsContext3D> m_context;
     OwnPtr<Extensions3DUtil> m_extensionsUtil;
     IntSize m_size;
     WebGraphicsContext3D::Attributes m_requestedAttributes;
     bool m_multisampleExtensionSupported;
     bool m_packedDepthStencilExtensionSupported;
     bool m_discardFramebufferSupported;
     Platform3DObject m_fbo;
     // DrawingBuffer's output is double-buffered. m_colorBuffer is the back buffer.
     TextureInfo m_colorBuffer;
-    TextureInfo m_frontColorBuffer;
+    struct FrontBufferInfo {
+        TextureInfo texInfo;
+        WebExternalTextureMailbox mailbox;
+    };
+    FrontBufferInfo m_frontColorBuffer;
 
     // This is used when we have OES_packed_depth_stencil.
     Platform3DObject m_depthStencilBuffer;
 
     // These are used when we don't.
     Platform3DObject m_depthBuffer;
     Platform3DObject m_stencilBuffer;
 
     // For multisampling.
     Platform3DObject m_multisampleFBO;
@@ skipping 40 matching lines @@
     // used to resize the Canvas.
     SkBitmap m_resizingBitmap;
 
     // Used to flip a bitmap vertically.
     Vector<uint8_t> m_scanline;
 };
 
 } // namespace blink
 
 #endif // DrawingBuffer_h