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..074f0b6349ba219464c23337fd39cf87159137cb 100644
--- a/content/renderer/input/render_widget_input_handler.cc
+++ b/content/renderer/input/render_widget_input_handler.cc
@@ -332,6 +332,19 @@ void RenderWidgetInputHandler::HandleInputEvent(
     LogPassiveEventListenersUma(
         processed, static_cast<const WebTouchEvent&>(input_event).dispatchType,
         input_event.timeStampSeconds, latency_info);
+
+    if (base::TimeTicks::IsHighResolution()) {
+      if (input_event.type == WebInputEvent::TouchStart &&
+          static_cast<const WebTouchEvent&>(input_event).dispatchType ==
+              WebInputEvent::Blocking &&
+          static_cast<const WebTouchEvent&>(input_event).shouldForceBePassive) {
+        base::TimeTicks now = base::TimeTicks::Now();
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Touch.ExperiencedForcedPassiveLatency",

[tdresser, 2016/05/06 18:49:19]

We'll want to record during fling and outside of fling, so we can compare them.

[lanwei, 2016/05/12 11:52:06]

Done.

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