Add metric to know how user use scrollbar

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

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
index ed1ffd697d43b82a43c5256fbfd81f78cd8d9364..8a41a30b5f4d7db42b8c009bb0b906b5a727405a 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
@@ -25,7 +25,7 @@
 
 #include "platform/scroll/Scrollbar.h"
 
-#include <algorithm>
+#include "platform/Histogram.h"
 #include "platform/HostWindow.h"
 #include "platform/PlatformMouseEvent.h"
 #include "platform/geometry/FloatRect.h"
@@ -317,6 +317,40 @@ void Scrollbar::setPressedPart(ScrollbarPart part) {
       || m_hoveredPart != NoPart)
     setNeedsPaintInvalidation(
         static_cast<ScrollbarPart>(m_pressedPart | m_hoveredPart | part));
+
+  // Send ScrollbarPressedPart histogram.
+  ScrollbarPressedPart scrollbarPressedPart = IgnorePart;

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

Put all of this into a helper method that you call from here.

+  switch (part) {
+    case BackButtonStartPart:
+    case ForwardButtonStartPart:
+    case BackButtonEndPart:
+    case ForwardButtonEndPart:
+      scrollbarPressedPart =
+          (orientation() == VerticalScrollbar ? VerticalScrollbarButton
+                                              : HorizontalScrollbarButton);
+      break;
+    case ThumbPart:
+      scrollbarPressedPart =
+          (orientation() == VerticalScrollbar ? VerticalScrollbarThumb
+                                              : HorizontalScrollbarThumb);
+      break;
+    case BackTrackPart:
+    case ForwardTrackPart:
+      scrollbarPressedPart =
+          (orientation() == VerticalScrollbar ? VerticalScrollbarTrack
+                                              : HorizontalScrollbarTrack);
+      break;
+    default:
+      scrollbarPressedPart = IgnorePart;
+  }
+
+  if (scrollbarPressedPart != IgnorePart) {
+    DEFINE_STATIC_LOCAL(EnumerationHistogram, scrollbarPressedPartHistogram,
+                        ("ScrollbarPressedPart", ScrollbarPressedPartMax));
+    scrollbarPressedPartHistogram.count(

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

This will still count multiple uses on a page load. See
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/frame/Vis...
how I made sure that we only report the viewport stats once per page load by
recording them at destruction time. (I'll confirm that we fixed the bug
regarding UMA stats collected at destruction)

+        static_cast<ScrollbarPressedPart>(scrollbarPressedPart));
+  }
+
   m_pressedPart = part;
 }