[VSync Queue] Add tracing for event queuing time and coalesced count

ui/events/blink/event_with_callback.h

 // Copyright 2016 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.
 
 #ifndef UI_EVENTS_BLINK_EVENT_WITH_CALLBACK_H_
 #define UI_EVENTS_BLINK_EVENT_WITH_CALLBACK_H_
 
 #include <list>
 
 #include "ui/events/blink/input_handler_proxy.h"
@@ skipping 37 matching lines @@
                     std::unique_ptr<DidOverscrollParams>);
 
   const blink::WebInputEvent& event() const { return *event_; }
   const LatencyInfo latency_info() const { return latency_; }
   base::TimeTicks creation_timestamp() const { return creation_timestamp_; }
   base::TimeTicks last_coalesced_timestamp() const {
     return last_coalesced_timestamp_;
   }
   size_t coalesced_count() const { return original_events_.size(); }
   OriginalEventList& original_events() { return original_events_; }
+  blink::WebInputEvent* first_original_event() {

[tdresser, 2017/02/16 13:44:18]

Can't we just use coalesced_count(), instead of adding a new method?

[chongz, 2017/02/16 15:08:58]

Added comments.

// |first_original_event()| is used as ID for tracing.

+    return original_events_.empty() ? nullptr
+                                    : original_events_.front().event_.get();
+  }
 
  private:
   friend class test::InputHandlerProxyEventQueueTest;
 
   void SetTickClockForTesting(std::unique_ptr<base::TickClock> tick_clock);
 
   WebScopedInputEvent event_;
   LatencyInfo latency_;
   OriginalEventList original_events_;
 
   base::TimeTicks creation_timestamp_;
   base::TimeTicks last_coalesced_timestamp_;
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_BLINK_EVENT_WITH_CALLBACK_H_