Allow DrawingBuffer to present to a bitmap.

public/platform/WebExternalTextureMailbox.h

 /*
  * Copyright (C) 2013 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 18 matching lines @@
  */
 
 #ifndef WebExternalTextureMailbox_h
 #define WebExternalTextureMailbox_h
 
 namespace WebKit {
 
 struct WebExternalTextureMailbox {
     signed char name[64];
     unsigned syncPoint;
+    SkBitmap bitmap;
+    WebSize size;
 
     WebExternalTextureMailbox()
         : syncPoint(0)
     {
         memset(name, 0, sizeof(name));
     }
 };
 
 } // namespace WebKit
 
 #endif // WebExternalTextureMailbox_h