Add UMA metric to track the time saved on making events passive during fling

content/renderer/input/render_widget_input_handler.cc

Index: content/renderer/input/render_widget_input_handler.cc
diff --git a/content/renderer/input/render_widget_input_handler.cc b/content/renderer/input/render_widget_input_handler.cc
index 75d8f46974b897e701baf2971508eefdd2fead98..ee6584e5ef5a8e99664187a7e876f1bab01897bb 100644
--- a/content/renderer/input/render_widget_input_handler.cc
+++ b/content/renderer/input/render_widget_input_handler.cc
@@ -332,6 +332,25 @@ void RenderWidgetInputHandler::HandleInputEvent(
     LogPassiveEventListenersUma(
         processed, static_cast<const WebTouchEvent&>(input_event).dispatchType,
         input_event.timeStampSeconds, latency_info);
+
+    if (base::TimeTicks::IsHighResolution() &&
+        input_event.type == WebInputEvent::TouchStart &&
+        static_cast<const WebTouchEvent&>(input_event).dispatchType ==
+            WebInputEvent::Blocking) {
+      base::TimeTicks now = base::TimeTicks::Now();
+      if (static_cast<const WebTouchEvent&>(input_event)

[tdresser, 2016/05/12 12:11:32]

Let's only perform the cast once, as soon as we know it's a touch event.

+              .dispatchedDuringFling) {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Touch.TouchStartDuringFlingLatency",
+            GetEventLatencyMicros(input_event.timeStampSeconds, now), 1,
+            10000000, 100);
+      } else {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Touch.TouchStartNoFlingLatency",

[tdresser, 2016/05/12 12:11:32]

TouchStartNoFlingLatency -> TouchStartOutsideFlingLatency, to be more parallel
to TouchStartDuringFlingLatency.

+            GetEventLatencyMicros(input_event.timeStampSeconds, now), 1,
+            10000000, 100);
+      }
+    }
   } else if (input_event.type == WebInputEvent::MouseWheel) {
     LogPassiveEventListenersUma(
         processed,