Record box shadow as main thread scrolling reasons

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
index 09662931475073ab2077ff2c2f53c418e49a8b56..9df31a040438e2f9a9e6830b1d316dd07db47d2a 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
@@ -197,7 +197,12 @@ class CORE_EXPORT LayoutBoxModelObject : public LayoutObject {
 
   // Returns which layers backgrounds should be painted into for overflow
   // scrolling boxes.
-  BackgroundPaintLocation backgroundPaintLocation() const;
+  // TODO(yigu): PaintLayerScrollableArea::computeNeedsCompositedScrolling may
+  // call this method directly 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;
 
   // These return the CSS computed padding values.
   LayoutUnit computedCSSPaddingTop() const {