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 6d05fe059f6f8a7ddb8b1370866f43cd5abda5ea..0c27322bc40a02cc94ee6566f92b891a910e07c6 100644
--- a/third_party/WebKit/Source/core/input/ScrollManager.cpp
+++ b/third_party/WebKit/Source/core/input/ScrollManager.cpp
@@ -235,6 +235,12 @@ 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;
 }