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

Source/platform/graphics/ImageBuffer.h

 /*
  * Copyright (C) 2006 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2007, 2008, 2009 Apple Inc. All rights reserved.
  * 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 106 matching lines @@
 
     // FIXME: current implementations of this method have the restriction that they only work
     // with textures that are RGB or RGBA format, UNSIGNED_BYTE type and level 0, as specified in
     // Extensions3D::canUseCopyTextureCHROMIUM().
     // Destroys the TEXTURE_2D binding for the active texture unit of the passed context
     bool copyToPlatformTexture(WebGraphicsContext3D*, Platform3DObject, GLenum, GLenum, GLint, bool, bool);
 
     Platform3DObject getBackingTexture();
     void didModifyBackingTexture();
 
-    bool copyRenderingResultsFromDrawingBuffer(DrawingBuffer*, bool fromFrontBuffer = false);
+    bool copyRenderingResultsFromDrawingBuffer(DrawingBuffer*, SourceDrawingBuffer);
 
     void flush();
 
     void notifySurfaceInvalid();
 
     PassRefPtr<SkImage> newImageSnapshot() const;
 
 private:
     ImageBuffer(PassOwnPtr<ImageBufferSurface>);
 
@@ skipping 19 matching lines @@
 
     IntSize m_size;
     RefPtr<Uint8ClampedArray> m_data;
 };
 
 String PLATFORM_EXPORT ImageDataToDataURL(const ImageDataBuffer&, const String& mimeType, const double* quality);
 
 } // namespace blink
 
 #endif // ImageBuffer_h