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

Source/core/html/canvas/WebGLRenderingContextBase.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 330 matching lines @@
     void forceRestoreContext();
     void loseContextImpl(LostContextMode, AutoRecoveryMethod);
 
     blink::WebGraphicsContext3D* webContext() const { return drawingBuffer()->context(); }
     WebGLContextGroup* contextGroup() const { return m_contextGroup.get(); }
     Extensions3DUtil* extensionsUtil();
 
     void reshape(int width, int height);
 
     void markLayerComposited();
-    PassRefPtrWillBeRawPtr<ImageData> paintRenderingResultsToImageData();
+    PassRefPtrWillBeRawPtr<ImageData> paintRenderingResultsToImageData(SourceBuffer);
 
     void removeSharedObject(WebGLSharedObject*);
     void removeContextObject(WebGLContextObject*);
 
     unsigned maxVertexAttribs() const { return m_maxVertexAttribs; }
 
     // ActiveDOMObject notifications
     virtual bool hasPendingActivity() const override;
     virtual void stop() override;
 
@@ skipping 590 matching lines @@
     static IntSize oldestContextSize();
 };
 
 DEFINE_TYPE_CASTS(WebGLRenderingContextBase, CanvasRenderingContext, context, context->is3d(), context.is3d());
 
 } // namespace blink
 
 WTF_ALLOW_MOVE_INIT_AND_COMPARE_WITH_MEM_FUNCTIONS(blink::WebGLRenderingContextBase::TextureUnitState);
 
 #endif // WebGLRenderingContextBase_h