Paint local background colors onto foreground layer.

third_party/WebKit/Source/core/paint/PaintLayer.h

Index: third_party/WebKit/Source/core/paint/PaintLayer.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.h b/third_party/WebKit/Source/core/paint/PaintLayer.h
index 469fe7891a25ab120649fccc0ca32b4ebda4c893..1c6da37f1cf821ecbdd397f17bdc9a070542c332 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.h
@@ -247,6 +247,13 @@ public:
     PaintLayerReflectionInfo* reflectionInfo() { return m_rareData ? m_rareData->reflectionInfo.get() : nullptr; }
     const PaintLayerReflectionInfo* reflectionInfo() const { return const_cast<PaintLayer*>(this)->reflectionInfo(); }
 
+    // Returns true for layers with scrollable overflow which have a background
+    // that can be painted into the composited scrolling contents layer (i.e.
+    // the background can scroll with the content). When the background is also
+    // opaque this allows us to composite the scroller even on low DPI as we can
+    // draw with subpixel anti-aliasing.
+    bool shouldPaintBackgroundOntoScrollingContentsLayer() const;

[chrishtr, 2016/08/05 00:40:08]

Move this method to CompositedLayerMapping.

[flackr, 2016/08/08 18:01:30]

Done.

+
     const PaintLayer* root() const
     {
         const PaintLayer* curr = this;