Add metric to know how user use scrollbar

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PaintInvalidationCapableScrollableArea_h
 #define PaintInvalidationCapableScrollableArea_h
 
 #include "core/CoreExport.h"
 #include "platform/geometry/LayoutRect.h"
 #include "platform/scroll/ScrollableArea.h"
@@ skipping 22 matching lines @@
 
   // Should be called when the previous visual rects are no longer valid.
   void clearPreviousVisualRects();
 
   virtual IntRect scrollCornerAndResizerRect() const {
     return scrollCornerRect();
   }
 
   LayoutRect visualRectForScrollbarParts() const override;
 
+  void didScrollWithScrollbar(ScrollbarPart, ScrollbarOrientation) override;
+
  private:
   virtual LayoutScrollbarPart* scrollCorner() const = 0;
   virtual LayoutScrollbarPart* resizer() const = 0;
 
   void scrollControlWasSetNeedsPaintInvalidation() override;
 
   bool m_horizontalScrollbarPreviouslyWasOverlay;
   bool m_verticalScrollbarPreviouslyWasOverlay;
   LayoutRect m_horizontalScrollbarPreviousVisualRect;
   LayoutRect m_verticalScrollbarPreviousVisualRect;
   LayoutRect m_scrollCornerAndResizerPreviousVisualRect;
 };
 
 }  // namespace blink
 
 #endif  // PaintInvalidationCapableScrollableArea_h