Paint solid color background into both scrolling contents layer and graphics layer.

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

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 485 matching lines @@
 
   // This returns true if our document is in a phase of its lifestyle during
   // which compositing state may legally be read.
   bool isAllowedToQueryCompositingState() const;
 
   // Don't null check this.
   // FIXME: Rename.
   CompositedLayerMapping* compositedLayerMapping() const;
   GraphicsLayer* graphicsLayerBacking() const;
   GraphicsLayer* graphicsLayerBackingForScrolling() const;
-  // Returns true for layers with scrollable overflow which have a background
-  // that can be painted into the composited scrolling contents layer when it
-  // exist (i.e. the background can scroll with the content). When the
-  // background is also opaque this allows us to composite the scroller even on
+  // Returns which layers backgrounds should be painted into for overflow

[chrishtr, 2016/11/17 20:55:39]

s/layers/GraphicsLayers

[flackr, 2016/11/17 23:38:00]

Done.

+  // scrolling boxes. When the background can paint into the scrolling contents
+  // and is also opaque this allows us to composite the scroller even on
   // low DPI as we can draw with subpixel anti-aliasing.
-  bool canPaintBackgroundOntoScrollingContentsLayer() const;
+  BackgroundPaintLocation backgroundPaintLocation() const;
   // NOTE: If you are using hasCompositedLayerMapping to determine the state of
   // compositing for this layer, (and not just to do bookkeeping related to the
   // mapping like, say, allocating or deallocating a mapping), then you may have
   // incorrect logic. Use compositingState() instead.
   // FIXME: This is identical to null checking compositedLayerMapping(), why not
   // just call that?
   bool hasCompositedLayerMapping() const {
     return m_rareData && m_rareData->compositedLayerMapping;
   }
   void ensureCompositedLayerMapping();
@@ skipping 701 matching lines @@
 
 }  // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 CORE_EXPORT void showLayerTree(const blink::PaintLayer*);
 CORE_EXPORT void showLayerTree(const blink::LayoutObject*);
 #endif
 
 #endif  // Layer_h