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

Source/core/frame/ImageBitmap.cpp

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/frame/ImageBitmap.h"
 
 #include "core/html/HTMLCanvasElement.h"
 #include "core/html/HTMLVideoElement.h"
 #include "core/html/ImageData.h"
@@ skipping 64 matching lines @@
     m_bitmapRect = IntRect(IntPoint(std::max(0, -cropRect.x()), std::max(0, -cropRect.y())), srcRect.size());
 }
 
 ImageBitmap::ImageBitmap(HTMLCanvasElement* canvas, const IntRect& cropRect)
     : m_imageElement(nullptr)
     , m_cropRect(cropRect)
     , m_bitmapOffset(IntPoint())
 {
     CanvasRenderingContext* sourceContext = canvas->renderingContext();
     if (sourceContext && sourceContext->is3d())
-        sourceContext->paintRenderingResultsToCanvas(CanvasRenderingContext::Back);
+        sourceContext->paintRenderingResultsToCanvas(BackBuffer);
 
     IntRect srcRect = intersection(cropRect, IntRect(IntPoint(), canvas->size()));
     m_bitmapRect = IntRect(IntPoint(std::max(0, -cropRect.x()), std::max(0, -cropRect.y())), srcRect.size());
     m_bitmap = cropImage(canvas->buffer()->copyImage(CopyBackingStore).get(), cropRect);
 }
 
 ImageBitmap::ImageBitmap(ImageData* data, const IntRect& cropRect)
     : m_imageElement(nullptr)
     , m_cropRect(cropRect)
     , m_bitmapOffset(IntPoint())
@@ skipping 121 matching lines @@
     return FloatSize(width(), height());
 }
 
 void ImageBitmap::trace(Visitor* visitor)
 {
     visitor->trace(m_imageElement);
     ImageLoaderClient::trace(visitor);
 }
 
 } // namespace blink