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

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 145 matching lines @@
     virtual void mailboxReleased(const WebExternalTextureMailbox&, bool lostResource = false) override;
 
     enum SourceBuffer { Front, Back };
     // Destroys the TEXTURE_2D binding for the owned context
     bool copyToPlatformTexture(WebGraphicsContext3D*, Platform3DObject texture, GLenum internalFormat,
         GLenum destType, GLint level, bool premultiplyAlpha, bool flipY, SourceBuffer);
 
     void setPackAlignment(GLint param);
 
     void paintRenderingResultsToCanvas(ImageBuffer*);
-    PassRefPtr<Uint8ClampedArray> paintRenderingResultsToImageData(int&, int&);
+    PassRefPtr<Uint8ClampedArray> paintRenderingResultsToImageData(int&, int&, SourceBuffer);
 
 protected: // For unittests
     DrawingBuffer(
         PassOwnPtr<WebGraphicsContext3D>,
         PassOwnPtr<Extensions3DUtil>,
         bool multisampleExtensionSupported,
         bool packedDepthStencilExtensionSupported,
         bool discardFramebufferSupported,
         PreserveDrawingBuffer,
         WebGraphicsContext3D::Attributes requestedAttributes,
@@ skipping 127 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