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

Source/platform/graphics/ImageBuffer.cpp

 /*
  * Copyright (c) 2008, Google Inc. All rights reserved.
  * Copyright (C) 2009 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. 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
@@ skipping 417 matching lines @@
 {
     ASSERT(MIMETypeRegistry::isSupportedImageMIMETypeForEncoding(mimeType));
 
     Vector<char> encodedImage;
     if (!isSurfaceValid() || !encodeImage(m_surface->bitmap(), mimeType, quality, &encodedImage))
         return "data:,";
 
     return "data:" + mimeType + ";base64," + base64Encode(encodedImage);
 }
 
-String ImageDataToDataURL(const ImageDataBuffer& imageData, const String& mimeType, const double* quality)
-{
-    ASSERT(MIMETypeRegistry::isSupportedImageMIMETypeForEncoding(mimeType));
-
-    Vector<char> encodedImage;
-    if (!encodeImage(imageData, mimeType, quality, &encodedImage))
-        return "data:,";
-
-    return "data:" + mimeType + ";base64," + base64Encode(encodedImage);
-}
-
 } // namespace blink