Blink API to allow mailbox usage with texture layers for WebGL and Canvas2D.

Source/Platform/Platform.gypi

 #
 # Copyright (C) 2011 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 37 matching lines @@
             'chromium/public/WebCompositorOutputSurface.h',
             'chromium/public/WebContentLayer.h',
             'chromium/public/WebContentLayerClient.h',
             'chromium/public/WebCookie.h',
             'chromium/public/WebCookieJar.h',
             'chromium/public/WebData.h',
             'chromium/public/WebDiscardableMemory.h',
             'chromium/public/WebDragData.h',
             'chromium/public/WebExternalTextureLayer.h',
             'chromium/public/WebExternalTextureLayerClient.h',
+            'chromium/public/WebExternalTextureMailbox.h',
             'chromium/public/WebFileError.h',
             'chromium/public/WebFileInfo.h',
             'chromium/public/WebFileSystem.h',
             'chromium/public/WebFileSystemCallbacks.h',
             'chromium/public/WebFileSystemEntry.h',
             'chromium/public/WebFileSystemType.h',
             'chromium/public/WebFileUtilities.h',
             'chromium/public/WebFilterOperation.h',
             'chromium/public/WebFilterOperations.h',
             'chromium/public/WebFlingAnimator.h',
@@ skipping 99 matching lines @@
             'chromium/src/WebActiveGestureAnimation.h',
             'chromium/src/WebCString.cpp',
             'chromium/src/WebFilterOperation.cpp',
             'chromium/src/WebFilterOperations.cpp',
             'chromium/src/WebFloatQuad.cpp',
             'chromium/src/WebPrerenderingSupport.cpp',
             'chromium/src/WebString.cpp',
         ]
     }
 }