UMA metrics for use count of wheel and touch scrolls.

third_party/WebKit/Source/core/input/ScrollManager.cpp

Index: third_party/WebKit/Source/core/input/ScrollManager.cpp
diff --git a/third_party/WebKit/Source/core/input/ScrollManager.cpp b/third_party/WebKit/Source/core/input/ScrollManager.cpp
index 3e6d4638681cfec8e6e42006dd88d629884d2035..7ab853793d1bf0b12a4f22641e664daa621654c1 100644
--- a/third_party/WebKit/Source/core/input/ScrollManager.cpp
+++ b/third_party/WebKit/Source/core/input/ScrollManager.cpp
@@ -236,6 +236,12 @@ WebInputEventResult ScrollManager::handleGestureScrollBegin(
       m_deltaConsumedForScrollSequence;
   ScrollState* scrollState = ScrollState::create(std::move(scrollStateData));
   customizedScroll(*m_scrollGestureHandlingNode.get(), *scrollState);
+
+  if (gestureEvent.sourceDevice == WebGestureDeviceTouchscreen)
+    UseCounter::count(m_frame->document(), UseCounter::ScrollByTouch);
+  else
+    UseCounter::count(m_frame->document(), UseCounter::ScrollByWheel);
+
   return WebInputEventResult::HandledSystem;
 }