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

Source/platform/graphics/ImageBuffer.cpp

 /*
  * Copyright (c) 2008, Google Inc. All rights reserved.
  * Copyright (C) 2009 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
@@ skipping 224 matching lines @@
 Platform3DObject ImageBuffer::getBackingTexture()
 {
     return m_surface->getBackingTexture();
 }
 
 void ImageBuffer::didModifyBackingTexture()
 {
     m_surface->didModifyBackingTexture();
 }
 
-bool ImageBuffer::copyRenderingResultsFromDrawingBuffer(DrawingBuffer* drawingBuffer, bool fromFrontBuffer)
+bool ImageBuffer::copyRenderingResultsFromDrawingBuffer(DrawingBuffer* drawingBuffer, SourceDrawingBuffer sourceBuffer)
 {
     if (!drawingBuffer)
         return false;
     OwnPtr<WebGraphicsContext3DProvider> provider = adoptPtr(Platform::current()->createSharedOffscreenGraphicsContext3DProvider());
     if (!provider)
         return false;
     WebGraphicsContext3D* context3D = provider->context3d();
     Platform3DObject tex = m_surface->getBackingTexture();
     if (!context3D || !tex)
         return false;
 
     m_surface->invalidateCachedBitmap();
     bool result = drawingBuffer->copyToPlatformTexture(context3D, tex, GL_RGBA,
-        GL_UNSIGNED_BYTE, 0, true, false, fromFrontBuffer ? DrawingBuffer::Front : DrawingBuffer::Back);
+        GL_UNSIGNED_BYTE, 0, true, false, sourceBuffer);
 
     if (result) {
         m_surface->didModifyBackingTexture();
     }
 
     return result;
 }
 
 void ImageBuffer::draw(GraphicsContext* context, const FloatRect& destRect, const FloatRect* srcPtr, CompositeOperator op, WebBlendMode blendMode)
 {
@@ skipping 177 matching lines @@
     ASSERT(MIMETypeRegistry::isSupportedImageMIMETypeForEncoding(mimeType));
 
     Vector<char> encodedImage;
     if (!encodeImage(imageData, mimeType, quality, &encodedImage))
         return "data:,";
 
     return "data:" + mimeType + ";base64," + base64Encode(encodedImage);
 }
 
 } // namespace blink