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

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 4a7719a0a71610db74226d475874e0c37b3d9eed..805f9d915efe14dd1211eca73f6ca6c1b3d49083 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.h
@@ -502,7 +502,12 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
   // 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