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

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

Index: Source/WebCore/platform/graphics/chromium/ScrollbarLayerChromium.cpp
===================================================================
--- Source/WebCore/platform/graphics/chromium/ScrollbarLayerChromium.cpp	(revision 120770)
+++ Source/WebCore/platform/graphics/chromium/ScrollbarLayerChromium.cpp	(working copy)
@@ -116,8 +116,6 @@
         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);
@@ -176,8 +174,6 @@
         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);