Treat calls to CanvasRenderingContext2D.clearRect as operations that clear the canvas

Source/platform/graphics/RecordingImageBufferSurface.cpp

 // Copyright 2014 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 "platform/graphics/RecordingImageBufferSurface.h"
 
 #include "platform/graphics/GraphicsContext.h"
 #include "platform/graphics/ImageBuffer.h"
@@ skipping 19 matching lines @@
 
 void RecordingImageBufferSurface::initializeCurrentFrame()
 {
     static SkRTreeFactory rTreeFactory;
     m_currentFrame = adoptPtr(new SkPictureRecorder);
     m_currentFrame->beginRecording(size().width(), size().height(), &rTreeFactory);
     m_initialSaveCount = m_currentFrame->getRecordingCanvas()->getSaveCount();
     if (m_graphicsContext) {
         m_graphicsContext->resetCanvas(m_currentFrame->getRecordingCanvas());
         m_graphicsContext->setTrackOpaqueRegion(true);
+        m_graphicsContext->setTreatOverwriteAsOpaque(true);
     }
 }
 
 void RecordingImageBufferSurface::setImageBuffer(ImageBuffer* imageBuffer)
 {
     m_graphicsContext = imageBuffer ? imageBuffer->context() : 0;
     if (m_currentFrame && m_graphicsContext) {
         m_graphicsContext->setTrackOpaqueRegion(true);
+        m_graphicsContext->setTreatOverwriteAsOpaque(true);
         m_graphicsContext->resetCanvas(m_currentFrame->getRecordingCanvas());
     }
 }
 
 void RecordingImageBufferSurface::willReadback()
 {
     fallBackToRasterCanvas();
 }
 
 void RecordingImageBufferSurface::fallBackToRasterCanvas()
@@ skipping 80 matching lines @@
 
     SkCanvas* newCanvas = m_currentFrame->getRecordingCanvas();
     newCanvas->concat(ctm);
     newCanvas->clipRect(clip);
 
     m_frameWasCleared = false;
     return true;
 }
 
 } // namespace WebCore