WebGL: clarify which Front or Back buffer is used by each API.

Source/platform/graphics/GraphicsTypes3D.h

 /*
  * 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:
  * 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 39 matching lines @@
 
 // GL_AMD_compressed_ATC_texture
 const unsigned GC3D_COMPRESSED_ATC_RGB_AMD = 0x8C92;
 const unsigned GC3D_COMPRESSED_ATC_RGBA_EXPLICIT_ALPHA_AMD = 0x8C93;
 const unsigned GC3D_COMPRESSED_ATC_RGBA_INTERPOLATED_ALPHA_AMD = 0x87EE;
 
 // GL_CHROMIUM_gpu_memory_buffer_image
 const unsigned GC3D_MAP_CHROMIUM = 0x78F1;
 const unsigned GC3D_SCANOUT_CHROMIUM = 0x78F2;
 
-#endif
+namespace blink {
+
+enum SourceDrawingBuffer { FrontBuffer, BackBuffer };
+
+} // namespace blink
+
+#endif // GraphicsTypes3D_h