Fix event_count calculation.

cc/debug/latency_info.cc

 // Copyright (c) 2013 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 "cc/debug/latency_info.h"
 
 #include <algorithm>
 
 namespace cc {
 
@@ skipping 30 matching lines @@
   } else {
     f->second.sequence_number = std::max(component_sequence_number,
                                          f->second.sequence_number);
     uint32 new_count = event_count + f->second.event_count;
     if (event_count > 0 && new_count != 0) {
       // Do a weighted average, so that the new event_time is the average of
       // the times of events currently in this structure with the time passed
       // into this method.
       f->second.event_time += (time - f->second.event_time) * event_count /
           new_count;
-      f->second.event_count += new_count;
+      f->second.event_count = new_count;
     }
   }
 }
 
 void LatencyInfo::Clear() {
   latency_components.clear();
 }
 
 } // namespace cc