Add more specific metrics for main thread scrolling reasons

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h

 /*
  * Copyright (C) 2003, 2009, 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>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 456 matching lines @@
     m_hadVerticalScrollbarBeforeRelayout = val;
   }
 
   StickyConstraintsMap& stickyConstraintsMap() {
     return ensureRareData().m_stickyConstraintsMap;
   }
   void invalidateAllStickyConstraints();
   void invalidateStickyConstraintsFor(PaintLayer*,
                                       bool needsCompositingUpdate = true);
 
+  void updateMainThreadScrollingReasonsFromStyle(const PaintLayer*) const;
+  bool hasMainThreadScrollingReason(uint32_t reason) const {
+    return m_reasons & reason;
+  }
+  void flipMainThreadScrollingReason(uint32_t reason) { m_reasons ^= reason; }
+
   uint64_t id() const;
 
   DECLARE_VIRTUAL_TRACE();
 
  private:
   explicit PaintLayerScrollableArea(PaintLayer&);
 
   bool hasHorizontalOverflow() const;
   bool hasVerticalOverflow() const;
   bool hasScrollableHorizontalOverflow() const;
@@ skipping 84 matching lines @@
   // LayoutObject to hold our custom scroll corner.
   LayoutScrollbarPart* m_scrollCorner;
 
   // LayoutObject to hold our custom resizer.
   LayoutScrollbarPart* m_resizer;
 
   ScrollAnchor m_scrollAnchor;
 
   std::unique_ptr<PaintLayerScrollableAreaRareData> m_rareData;
 
+  // MainThreadScrollingReason due to the properties of the LayoutObject
+  uint32_t m_reasons;
+
 #if ENABLE(ASSERT)
   bool m_hasBeenDisposed;
 #endif
 };
 
 DEFINE_TYPE_CASTS(PaintLayerScrollableArea,
                   ScrollableArea,
                   scrollableArea,
                   scrollableArea->isPaintLayerScrollableArea(),
                   scrollableArea.isPaintLayerScrollableArea());
 
 }  // namespace blink
 
 #endif  // LayerScrollableArea_h