Make first TouchStart and first TouchMove events on a flinging layer non-blocking

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 55f2aefb8063b0a2623e71ee0f0e57b693fa37d3..a77f9d1ff2825dd64ab3c5dcea0b40d43cd3b85b 100644
--- a/content/renderer/input/render_widget_input_handler.cc
+++ b/content/renderer/input/render_widget_input_handler.cc
@@ -127,14 +127,14 @@ void LogPassiveEventListenersUma(WebInputEventResult result,
     PASSIVE_LISTENER_UMA_ENUM_SUPPRESSED,
     PASSIVE_LISTENER_UMA_ENUM_CANCELABLE,
     PASSIVE_LISTENER_UMA_ENUM_CANCELABLE_AND_CANCELED,
-    PASSIVE_LISTENER_UMA_ENUM_FORCED_NON_BLOCKING,
+    PASSIVE_LISTENER_UMA_ENUM_FORCED_NON_BLOCKING_DUE_TO_FLING,
     PASSIVE_LISTENER_UMA_ENUM_COUNT
   };
 
   int enum_value;
   switch (dispatch_type) {
-    case WebInputEvent::ListenersForcedNonBlockingPassive:
-      enum_value = PASSIVE_LISTENER_UMA_ENUM_FORCED_NON_BLOCKING;
+    case WebInputEvent::ListenersForcedNonBlockingPassiveDueToFling:
+      enum_value = PASSIVE_LISTENER_UMA_ENUM_FORCED_NON_BLOCKING_DUE_TO_FLING;
       break;
     case WebInputEvent::ListenersNonBlockingPassive:
       enum_value = PASSIVE_LISTENER_UMA_ENUM_PASSIVE;
@@ -164,10 +164,11 @@ void LogPassiveEventListenersUma(WebInputEventResult result,
       UMA_HISTOGRAM_CUSTOM_COUNTS("Event.PassiveListeners.Latency",
                                   GetEventLatencyMicros(event_timestamp, now),
                                   1, 10000000, 100);
-    } else if (enum_value == PASSIVE_LISTENER_UMA_ENUM_FORCED_NON_BLOCKING) {
+    } else if (enum_value ==
+               PASSIVE_LISTENER_UMA_ENUM_FORCED_NON_BLOCKING_DUE_TO_FLING) {
       base::TimeTicks now = base::TimeTicks::Now();
       UMA_HISTOGRAM_CUSTOM_COUNTS(
-          "Event.PassiveListeners.ForcedNonBlockingLatency",
+          "Event.PassiveListeners.ForcedNonBlockingLatencyDueToFling",
           GetEventLatencyMicros(event_timestamp, now), 1, 10000000, 100);
     }
   }
@@ -334,21 +335,16 @@ void RenderWidgetInputHandler::HandleInputEvent(
     LogPassiveEventListenersUma(processed, touch.dispatchType,
                                 input_event.timeStampSeconds, latency_info);
 
-    if (input_event.type == WebInputEvent::TouchStart &&
-        touch.dispatchType == WebInputEvent::Blocking &&
+    // TODO(lanwei): May remove it when we no longer need any metric for event

[tdresser, 2016/08/17 14:32:28]

Make it clear what "it" is here.
Make this a bit more concrete, with a bug.

e.g., 

"TODO(lanwei): Remove this metric in M5?, once we've gathered enough data to
..."

[lanwei, 2016/08/18 12:31:47]

Done.

+    // latency outside fling.
+    if (touch.dispatchType == WebInputEvent::Blocking &&
+        touch.touchStartOrFirstTouchMove &&
         base::TimeTicks::IsHighResolution()) {
       base::TimeTicks now = base::TimeTicks::Now();
-      if (touch.dispatchedDuringFling) {
-        UMA_HISTOGRAM_CUSTOM_COUNTS(
-            "Event.Touch.TouchStartLatencyDuringFling",
-            GetEventLatencyMicros(input_event.timeStampSeconds, now), 1,
-            100000000, 50);
-      } else {
-        UMA_HISTOGRAM_CUSTOM_COUNTS(
-            "Event.Touch.TouchStartLatencyOutsideFling",
-            GetEventLatencyMicros(input_event.timeStampSeconds, now), 1,
-            100000000, 50);
-      }
+      UMA_HISTOGRAM_CUSTOM_COUNTS(
+          "Event.Touch.TouchLatencyOutsideFling",
+          GetEventLatencyMicros(input_event.timeStampSeconds, now), 1,
+          100000000, 50);
     }
   } else if (input_event.type == WebInputEvent::MouseWheel) {
     LogPassiveEventListenersUma(