Reland "Box shadow should be recorded as main thread scrolling reasons for non-root layers."

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 1ccc905bee7a665be7dc5e495c3ab7f25d6ab480..f9fc1a680e963c6d0bb246aba769b39516716bcd 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.h
@@ -501,7 +501,12 @@
   // 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