Record box shadow as main thread scrolling reasons

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 e9ead47d815e459aea27b981acc89a949c5d4fd0..f57327cb49e3d2069ecee19945971cf364253502 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.h
@@ -503,7 +503,12 @@ 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