Record GCM received data message intervals in UMA

components/gcm_driver/gcm_stats_recorder_impl.h

Index: components/gcm_driver/gcm_stats_recorder_impl.h
diff --git a/components/gcm_driver/gcm_stats_recorder_impl.h b/components/gcm_driver/gcm_stats_recorder_impl.h
index 4c3c57ec55ab69f7cbf58ce778c91353c4c65fb7..f962b4fe49a34e49dad6e43127518b39c0e8066f 100644
--- a/components/gcm_driver/gcm_stats_recorder_impl.h
+++ b/components/gcm_driver/gcm_stats_recorder_impl.h
@@ -9,6 +9,7 @@
 #include <string>
 #include <vector>
 
+#include "base/time/default_clock.h"
 #include "base/time/time.h"
 #include "components/gcm_driver/gcm_activity.h"
 #include "google_apis/gcm/engine/connection_factory.h"
@@ -147,6 +148,9 @@ class GCMStatsRecorderImpl : public GCMStatsRecorder {
   std::deque<ReceivingActivity> receiving_activities_;
   std::deque<SendingActivity> sending_activities_;
 
+  scoped_ptr<base::Clock> clock_;
+  int64 last_data_message_received_timestamp_;
+
   DISALLOW_COPY_AND_ASSIGN(GCMStatsRecorderImpl);
 };