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

Source/core/editing/Editor.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  *
  * 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
@@ skipping 411 matching lines @@
 }
 
 static Image* imageFromNode(const Node& node)
 {
     node.document().updateLayoutIgnorePendingStylesheets();
     RenderObject* renderer = node.renderer();
     if (!renderer)
         return nullptr;
 
     if (renderer->isCanvas())
-        return toHTMLCanvasElement(node).copiedImage();
+        return toHTMLCanvasElement(node).copiedImage(HTMLCanvasElement::Front);
 
     if (renderer->isImage()) {
         RenderImage* renderImage = toRenderImage(renderer);
         if (!renderImage)
             return nullptr;
 
         ImageResource* cachedImage = renderImage->cachedImage();
         if (!cachedImage || cachedImage->errorOccurred())
             return nullptr;
         return cachedImage->imageForRenderer(renderImage);
@@ skipping 836 matching lines @@
 }
 
 void Editor::trace(Visitor* visitor)
 {
     visitor->trace(m_frame);
     visitor->trace(m_lastEditCommand);
     visitor->trace(m_mark);
 }
 
 } // namespace blink