Add metric to know how user use scrollbar

third_party/WebKit/Source/platform/scroll/Scrollbar.h

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.h
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.h b/third_party/WebKit/Source/platform/scroll/Scrollbar.h
index a9cf98a131a2324eda0fa4c463733d68ebab51a1..ab7a56a5608f8d64024d4430dd1a4bedacf30326 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.h
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.h
@@ -44,6 +44,17 @@ class ScrollableArea;
 class ScrollbarTheme;
 class WebGestureEvent;
 
+enum ScrollbarPressedPart {

[bokan, 2017/01/16 21:25:33]

This should reflect the fact that it's used only for metrics. I'd name this
ScrollbarUseUMA

+  IgnorePart = 0,
+  VerticalScrollbarButton,
+  HorizontalScrollbarButton,
+  VerticalScrollbarThumb,

[bokan, 2017/01/16 21:25:33]

Nit: I think it be better to group all the horizontal and vertical ones
together:

VerticalScrollbarButton,
VerticalScrollbarThumb,
etc

+  HorizontalScrollbarThumb,
+  VerticalScrollbarTrack,
+  HorizontalScrollbarTrack,
+  ScrollbarPressedPartMax
+};
+
 class PLATFORM_EXPORT Scrollbar : public Widget,
                                   public ScrollbarThemeClient,
                                   public DisplayItemClient {