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..a2d6e657f103c21b092dab4d0e04f1602ff677cc 100644
--- a/content/renderer/input/render_widget_input_handler.cc
+++ b/content/renderer/input/render_widget_input_handler.cc
@@ -329,9 +329,27 @@ void RenderWidgetInputHandler::HandleInputEvent(
   if (input_event.type == WebInputEvent::TouchStart ||
       input_event.type == WebInputEvent::TouchMove ||
       input_event.type == WebInputEvent::TouchEnd) {
-    LogPassiveEventListenersUma(
-        processed, static_cast<const WebTouchEvent&>(input_event).dispatchType,
-        input_event.timeStampSeconds, latency_info);
+    WebTouchEvent touch = static_cast<const WebTouchEvent&>(input_event);

[tdresser, 2016/05/13 13:32:59]

Make this a const ref.

[lanwei, 2016/05/17 22:51:37]

Done.

+
+    LogPassiveEventListenersUma(processed, touch.dispatchType,
+                                input_event.timeStampSeconds, latency_info);
+
+    if (input_event.type == WebInputEvent::TouchStart &&
+        touch.dispatchType == WebInputEvent::Blocking &&
+        base::TimeTicks::IsHighResolution()) {
+      base::TimeTicks now = base::TimeTicks::Now();
+      if (touch.dispatchedDuringFling) {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Touch.TouchStartDuringFlingLatency",
+            GetEventLatencyMicros(input_event.timeStampSeconds, now), 1,
+            10000000, 100);

[Ilya Sherman, 2016/05/17 01:27:29]

Would 50 buckets suffice here?

[lanwei, 2016/05/17 22:51:37]

Yes.

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

[Ilya Sherman, 2016/05/17 01:27:29]

Ditto

+      }
+    }
   } else if (input_event.type == WebInputEvent::MouseWheel) {
     LogPassiveEventListenersUma(
         processed,