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(SourceDrawingBuffer);
 
     void removeSharedObject(WebGLSharedObject*);
     void removeContextObject(WebGLContextObject*);
 
     unsigned maxVertexAttribs() const { return m_maxVertexAttribs; }
 
     // ActiveDOMObject notifications
     virtual bool hasPendingActivity() const override;
     virtual void stop() override;
 
@@ skipping 29 matching lines @@
     void setupFlags();
 
 #if ENABLE(OILPAN)
     PassRefPtr<WebGLSharedWebGraphicsContext3D> sharedWebGraphicsContext3D() const;
 #endif
 
     // CanvasRenderingContext implementation.
     virtual bool is3d() const override { return true; }
     virtual bool isAccelerated() const override { return true; }
     virtual void setIsHidden(bool) override;
-    virtual void paintRenderingResultsToCanvas(SourceBuffer) override;
+    virtual void paintRenderingResultsToCanvas(SourceDrawingBuffer) override;
     virtual blink::WebLayer* platformLayer() const override;
 
     void addSharedObject(WebGLSharedObject*);
     void addContextObject(WebGLContextObject*);
     void detachAndRemoveAllObjects();
 
     void destroyContext();
     void markContextChanged(ContentChangeType);
 
     // Query if the GL implementation is NPOT strict.
@@ skipping 540 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