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

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

Index: Source/WebCore/platform/graphics/chromium/CanvasLayerTextureUpdater.cpp
===================================================================
--- Source/WebCore/platform/graphics/chromium/CanvasLayerTextureUpdater.cpp	(revision 120770)
+++ Source/WebCore/platform/graphics/chromium/CanvasLayerTextureUpdater.cpp	(working copy)
@@ -33,6 +33,9 @@
 #include "GraphicsContext.h"
 #include "LayerPainterChromium.h"
 #include "PlatformContextSkia.h"
+#include "SkCanvas.h"
+#include "SkPaint.h"
+#include "SkRect.h"
 #include "SkiaUtils.h"
 #include "TraceEvent.h"
 
@@ -63,6 +66,12 @@
         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();