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

content/renderer/input/render_widget_input_handler.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/input/render_widget_input_handler.h"
 
 #include <stddef.h>
 #include <stdint.h>
 #include <utility>
 
@@ skipping 314 matching lines @@
 
   // TODO(dtapuska): Use the input_event.timeStampSeconds as the start
   // ideally this should be when the event was sent by the compositor to the
   // renderer. crbug.com/565348
   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);
+
+    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,
         static_cast<const WebMouseWheelEvent&>(input_event).dispatchType,
         input_event.timeStampSeconds, latency_info);
   }
 
   // If this RawKeyDown event corresponds to a browser keyboard shortcut and
   // it's not processed by webkit, then we need to suppress the upcoming Char
   // events.
@@ skipping 202 matching lines @@
   if (pending_input_event_ack_) {
     TRACE_EVENT_ASYNC_END0("input",
                            "RenderWidgetInputHandler::ThrottledInputEventAck",
                            pending_input_event_ack_.get());
     delegate_->OnInputEventAck(std::move(pending_input_event_ack_));
   }
   total_input_handling_time_this_frame_ = base::TimeDelta();
 }
 
 }  // namespace content