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

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

 /*
  * Copyright (C) 2012 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
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 98 matching lines @@
     static PassOwnPtr<ScrollbarBackgroundPainter> create(ScrollbarThemeClient* scrollbar, ScrollbarThemeComposite* theme)
     {
         return adoptPtr(new ScrollbarBackgroundPainter(scrollbar, theme));
     }
 
     virtual void paint(SkCanvas* canvas, const IntRect& contentRect, IntRect&) OVERRIDE
     {
         PlatformContextSkia platformContext(canvas);
         platformContext.setDrawingToImageBuffer(true);
         GraphicsContext context(&platformContext);
-        context.clearRect(contentRect);
-        context.clip(contentRect);
 
         // The following is a simplification of ScrollbarThemeComposite::paint.
         m_theme->paintScrollbarBackground(&context, m_scrollbar);
 
         if (m_theme->hasButtons(m_scrollbar)) {
             IntRect backButtonStartPaintRect = m_theme->backButtonRect(m_scrollbar, BackButtonStartPart, true);
             m_theme->paintButton(&context, m_scrollbar, backButtonStartPaintRect, BackButtonStartPart);
 
             IntRect backButtonEndPaintRect = m_theme->backButtonRect(m_scrollbar, BackButtonEndPart, true);
             m_theme->paintButton(&context, m_scrollbar, backButtonEndPaintRect, BackButtonEndPart);
@@ skipping 38 matching lines @@
     {
         return adoptPtr(new ScrollbarThumbPainter(scrollbar, theme));
     }
 
     virtual void paint(SkCanvas* canvas, const IntRect& contentRect, IntRect& opaque) OVERRIDE
     {
         PlatformContextSkia platformContext(canvas);
         platformContext.setDrawingToImageBuffer(true);
         GraphicsContext context(&platformContext);
 
-        context.clearRect(contentRect);
-
         // Consider the thumb to be at the origin when painting.
         IntRect thumbRect = IntRect(IntPoint(), m_theme->thumbRect(m_scrollbar).size());
         m_theme->paintThumb(&context, m_scrollbar, thumbRect);
     }
 
 private:
     ScrollbarThumbPainter(ScrollbarThemeClient* scrollbar, ScrollbarThemeComposite* theme)
         : m_scrollbar(scrollbar)
         , m_theme(theme)
     {
@@ skipping 69 matching lines @@
     updatePart(m_backgroundUpdater.get(), m_background.get(), contentRect, updater);
 
     // Consider the thumb to be at the origin when painting.
     IntRect thumbRect = IntRect(IntPoint(), theme()->thumbRect(m_scrollbar.get()).size());
     if (!thumbRect.isEmpty())
         updatePart(m_thumbUpdater.get(), m_thumb.get(), thumbRect, updater);
 }
 
 }
 #endif // USE(ACCELERATED_COMPOSITING)