Merge 120640 - [chromium] REGRESSION(r120346) ContentLayerPainter should clear rect to be painted

Source/WebCore/platform/graphics/chromium/CanvasLayerTextureUpdater.cpp

 /*
  * Copyright (C) 2011 Google 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
@@ skipping 15 matching lines @@
 
 #include "config.h"
 
 #if USE(ACCELERATED_COMPOSITING)
 
 #include "CanvasLayerTextureUpdater.h"
 
 #include "GraphicsContext.h"
 #include "LayerPainterChromium.h"
 #include "PlatformContextSkia.h"
+#include "SkCanvas.h"
+#include "SkPaint.h"
+#include "SkRect.h"
 #include "SkiaUtils.h"
 #include "TraceEvent.h"
 
 namespace WebCore {
 
 CanvasLayerTextureUpdater::CanvasLayerTextureUpdater(PassOwnPtr<LayerPainterChromium> painter)
     : m_painter(painter)
 {
 }
 
@@ skipping 10 matching lines @@
     IntRect scaledContentRect = contentRect;
 
     if (contentsScale != 1.0) {
         canvas->scale(WebCoreFloatToSkScalar(contentsScale), WebCoreFloatToSkScalar(contentsScale));
 
         FloatRect rect = contentRect;
         rect.scale(1 / contentsScale);
         scaledContentRect = enclosingIntRect(rect);
     }
 
+    SkPaint paint;
+    paint.setAntiAlias(false);
+    paint.setXfermodeMode(SkXfermode::kClear_Mode);
+    canvas->drawRect(scaledContentRect, paint);
+    canvas->clipRect(scaledContentRect);
+
     m_painter->paint(canvas, scaledContentRect, resultingOpaqueRect);
     canvas->restore();
 
     m_contentRect = contentRect;
 }
 
 } // namespace WebCore
 #endif // USE(ACCELERATED_COMPOSITING)