Add metric to determine the benefit of forcing an event listener to be passive.

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 3d793bb66e161b15a01cc1f6732cdcf50c3fe820..5ac461362c7c5b66423a61d757ae1f3df0eaac8a 100644
--- a/content/renderer/input/render_widget_input_handler.cc
+++ b/content/renderer/input/render_widget_input_handler.cc
@@ -117,11 +117,6 @@ void LogInputEventLatencyUma(const WebInputEvent& event, base::TimeTicks now) {
 #undef CASE_TYPE
 }
 
-void LogPassiveLatency(int64_t latency) {
-  UMA_HISTOGRAM_CUSTOM_COUNTS("Event.PassiveListeners.Latency", latency, 1,
-                              10000000, 100);
-}
-
 void LogPassiveEventListenersUma(WebInputEventResult result,
                                  WebInputEvent::DispatchType dispatch_type,
                                  double event_timestamp,
@@ -166,7 +161,16 @@ void LogPassiveEventListenersUma(WebInputEventResult result,
   if (enum_value == PASSIVE_LISTENER_UMA_ENUM_CANCELABLE &&
       base::TimeTicks::IsHighResolution()) {
     base::TimeTicks now = base::TimeTicks::Now();
-    LogPassiveLatency(GetEventLatencyMicros(event_timestamp, now));
+    UMA_HISTOGRAM_CUSTOM_COUNTS("Event.PassiveListeners.Latency",
+                                GetEventLatencyMicros(event_timestamp, now), 1,
+                                10000000, 100);
+  }
+
+  if (dispatch_type == WebInputEvent::ListenersForcedNonBlockingPassive) {
+    base::TimeTicks now = base::TimeTicks::Now();

[tdresser, 2016/04/21 13:36:42]

Are you intentionally only recording the first one if it's high resolution, but
always recording this metric?

Also, we can't have both Event.PassiveListeners.Latency and
Event.PassiveListeners.ForcedNonBlockingLatency, can we?

Maybe move this to an else if, or add a DCHECK or something?

[dtapuska, 2016/04/21 13:38:34]

Err no; I meant to move this into a if(highRes) {
...
}

[dtapuska, 2016/04/21 20:46:38]

Done.

+    UMA_HISTOGRAM_CUSTOM_COUNTS(
+        "Event.PassiveListeners.ForcedNonBlockingLatency",
+        GetEventLatencyMicros(event_timestamp, now), 1, 10000000, 100);
   }
 }