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

Source/core/html/HTMLCanvasElement.h

 /*
  * Copyright (C) 2004, 2006, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * 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 102 matching lines @@
 
     void paint(GraphicsContext*, const LayoutRect&);
 
     GraphicsContext* drawingContext() const;
     GraphicsContext* existingDrawingContext() const;
 
     CanvasRenderingContext* renderingContext() const { return m_context.get(); }
 
     void ensureUnacceleratedImageBuffer();
     ImageBuffer* buffer() const;
-    Image* copiedImage() const;
+    enum SourceBuffer { Front, Back };
+    Image* copiedImage(SourceBuffer) const;
     void clearCopiedImage();
-    PassRefPtrWillBeRawPtr<ImageData> getImageData() const;
 
     SecurityOrigin* securityOrigin() const;
     bool originClean() const { return m_originClean; }
     void setOriginTainted() { m_originClean = false; }
 
     AffineTransform baseTransform() const;
 
     bool is3D() const;
 
     bool hasImageBuffer() const { return m_imageBuffer; }
@@ skipping 45 matching lines @@
     bool shouldUseDisplayList(const IntSize& deviceSize);
 
     void resetDirtyRect();
 
     void setSurfaceSize(const IntSize&);
 
     bool paintsIntoCanvasBuffer() const;
 
     void updateExternallyAllocatedMemory() const;
 
-    String toDataURLInternal(const String& mimeType, const double* quality, bool isSaving = false) const;
+    String toDataURLInternal(const String& mimeType, const double* quality, SourceBuffer) const;
 
     WillBeHeapHashSet<RawPtrWillBeWeakMember<CanvasObserver>> m_observers;
 
     IntSize m_size;
 
     OwnPtrWillBeMember<CanvasRenderingContext> m_context;
 
     bool m_ignoreReset;
     bool m_accelerationDisabled;
     FloatRect m_dirtyRect;
 
     mutable intptr_t m_externallyAllocatedMemory;
 
     bool m_originClean;
 
     // It prevents HTMLCanvasElement::buffer() from continuously re-attempting to allocate an imageBuffer
     // after the first attempt failed.
     mutable bool m_didFailToCreateImageBuffer;
     mutable bool m_didClearImageBuffer;
     OwnPtr<ImageBuffer> m_imageBuffer;
     mutable OwnPtr<GraphicsContextStateSaver> m_contextStateSaver;
 
     mutable RefPtr<Image> m_copiedImage; // FIXME: This is temporary for platforms that have to copy the image buffer to render (and for CSSCanvasValue).
 };
 
 } // namespace blink
 
 #endif // HTMLCanvasElement_h