Reland recording box shadow as main thread scrolling reason with conflict fixed

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 484 matching lines @@
 
   // Returns the GraphicsLayer owned by this PaintLayer's
   // CompositedLayerMapping (or groupedMapping()'s, if squashed),
   // into which the given LayoutObject paints. If null, assumes the
   // LayoutObject is *not* layoutObject().
   // Assumes that the given LayoutObject paints into one of the GraphicsLayers
   // associated with this PaintLayer.
   // Returns nullptr if this PaintLayer is not composited.
   GraphicsLayer* graphicsLayerBacking(const LayoutObject* = nullptr) const;
 
-  BackgroundPaintLocation backgroundPaintLocation() const;
+  // TODO(yigu): PaintLayerScrollableArea::computeNeedsCompositedScrolling
+  // calls this method to obtain main thread scrolling reasons due to
+  // background paint location. Once the cases get handled on compositor the
+  // parameter "reasons" could be removed.
+  BackgroundPaintLocation backgroundPaintLocation(
+      uint32_t* reasons = nullptr) 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 735 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