Add metric to know how user use scrollbar

third_party/WebKit/Source/platform/Widget.h

Index: third_party/WebKit/Source/platform/Widget.h
diff --git a/third_party/WebKit/Source/platform/Widget.h b/third_party/WebKit/Source/platform/Widget.h
index 2c59cb0a6a31dd3addb94e191feb1321aef6d069..f22f7bc6700727fbae5053f5475dde178454085e 100644
--- a/third_party/WebKit/Source/platform/Widget.h
+++ b/third_party/WebKit/Source/platform/Widget.h
@@ -32,6 +32,7 @@
 #include "platform/geometry/FloatPoint.h"
 #include "platform/geometry/IntRect.h"
 #include "platform/heap/Handle.h"
+#include "platform/scroll/ScrollTypes.h"
 #include "public/platform/WebFocusType.h"
 #include "wtf/Forward.h"
 
@@ -126,6 +127,8 @@ class PLATFORM_EXPORT Widget : public GarbageCollectedFinalized<Widget> {
   DECLARE_VIRTUAL_TRACE();
   virtual void dispose() {}
 
+  virtual void sendScrollbarUseUMA(ScrollbarPart, ScrollbarOrientation) {}

[bokan, 2017/01/17 21:24:01]

Rather than going through widget, I'd add a
ScrollableArea::didScrollWithScrollbar and implement it in
PaintInvalidationCapableScrollableArea which covers both
PaintLayerScrollableArea and FrameView.

+
  private:
   Member<Widget> m_parent;
   IntRect m_frameRect;