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

Source/core/html/canvas/CanvasRenderingContext.h

 /*
  * Copyright (C) 2009 Apple 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 40 matching lines @@
 #endif
 
     HTMLCanvasElement* canvas() const { return m_canvas; }
 
     virtual bool is2d() const { return false; }
     virtual bool is3d() const { return false; }
     virtual bool isAccelerated() const { return false; }
     virtual bool hasAlpha() const { return true; }
     virtual void setIsHidden(bool) = 0;
 
-    enum SourceBuffer { Front, Back };
-    virtual void paintRenderingResultsToCanvas(SourceBuffer) { }
+    virtual void paintRenderingResultsToCanvas(SourceDrawingBuffer) { }
 
     virtual blink::WebLayer* platformLayer() const { return nullptr; }
 
     virtual void trace(Visitor* visitor) { visitor->trace(m_canvas); }
 
     bool wouldTaintOrigin(CanvasImageSource*);
 
 protected:
     CanvasRenderingContext(HTMLCanvasElement*);
 
 private:
     RawPtrWillBeMember<HTMLCanvasElement> m_canvas;
     HashSet<String> m_cleanURLs;
     HashSet<String> m_dirtyURLs;
 };
 
 } // namespace blink
 
 #endif