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

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. 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 2578 matching lines @@
     case PaintsIntoGroupedBacking:
       return groupedMapping()->squashingLayer();
     default:
       return (obj != layoutObject() &&
               compositedLayerMapping()->scrollingContentsLayer())
                  ? compositedLayerMapping()->scrollingContentsLayer()
                  : compositedLayerMapping()->mainGraphicsLayer();
   }
 }
 
-BackgroundPaintLocation PaintLayer::backgroundPaintLocation() const {
+BackgroundPaintLocation PaintLayer::backgroundPaintLocation(
+    uint32_t* reasons) const {
   BackgroundPaintLocation location;
   if (!scrollsOverflow()) {
     location = BackgroundPaintInGraphicsLayer;
   } else if (RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
-    location = layoutObject()->backgroundPaintLocation();
+    location = layoutObject()->backgroundPaintLocation(reasons);
   } else {
     location = isRootLayer() ? BackgroundPaintInGraphicsLayer
-                             : layoutObject()->backgroundPaintLocation();
+                             : layoutObject()->backgroundPaintLocation(reasons);
   }
   m_stackingNode->updateLayerListsIfNeeded();
   if (m_stackingNode->hasNegativeZOrderList())
     location = BackgroundPaintInGraphicsLayer;
   return location;
 }
 
 void PaintLayer::ensureCompositedLayerMapping() {
   if (m_rareData && m_rareData->compositedLayerMapping)
     return;
@@ skipping 599 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->enclosingLayer());
 }
 #endif