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..3323ab82b367286b817e67791a2de84a305aed62 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"
@@ -317,9 +317,47 @@ void Scrollbar::setPressedPart(ScrollbarPart part) {
       || m_hoveredPart != NoPart)
     setNeedsPaintInvalidation(
         static_cast<ScrollbarPart>(m_pressedPart | m_hoveredPart | part));
+
+  sendScrollbarUseUMA(part);
   m_pressedPart = part;
 }
 
+void Scrollbar::sendScrollbarUseUMA(ScrollbarPart part) {
+  DCHECK(root());
+
+  ScrollbarUseUMA scrollbarUseUMA = IgnorePart;
+  switch (part) {
+    case BackButtonStartPart:
+    case ForwardButtonStartPart:
+    case BackButtonEndPart:
+    case ForwardButtonEndPart:
+      scrollbarUseUMA =
+          (orientation() == VerticalScrollbar ? VerticalScrollbarButton
+                                              : HorizontalScrollbarButton);
+      break;
+    case ThumbPart:
+      scrollbarUseUMA =
+          (orientation() == VerticalScrollbar ? VerticalScrollbarThumb
+                                              : HorizontalScrollbarThumb);
+      break;
+    case BackTrackPart:
+    case ForwardTrackPart:
+      scrollbarUseUMA =
+          (orientation() == VerticalScrollbar ? VerticalScrollbarTrack
+                                              : HorizontalScrollbarTrack);
+      break;
+    default:
+      scrollbarUseUMA = IgnorePart;
+  }
+
+  if (scrollbarUseUMA != IgnorePart &&
+      root()->shouldSendScrollbarUseUMA(scrollbarUseUMA)) {
+    DEFINE_STATIC_LOCAL(EnumerationHistogram, scrollbarUseHistogram,
+                        ("Scrollbar.ScrollbarUse", ScrollbarUseUMAMax));
+    scrollbarUseHistogram.count(static_cast<ScrollbarUseUMA>(part));
+  }
+}
+
 bool Scrollbar::gestureEvent(const WebGestureEvent& evt,
                              bool* shouldUpdateCapture) {
   DCHECK(shouldUpdateCapture);