Add PendingEvents UMA to ConnectionEventTracker shutdown.

Currently, the ConnectionEventTracker keeps a buffer in memory of events
which still need to be sent to GCM. If there is a shutdown, those events
are discarded without reaching GCM. This CL adds a metric to track the
event count at shutdown to give an estimate of how many events are being
lost and whether a persistence layer needs to be added.

BUG=662983

Review-Url: https://codereview.chromium.org/2594373002
Cr-Commit-Position: refs/heads/master@{#443208}

google_apis/gcm/engine/connection_event_tracker.cc

@@ -4,6 +4,7 @@
 
 #include "google_apis/gcm/engine/connection_event_tracker.h"
 
+#include "base/metrics/histogram_macros.h"
 #include "base/time/time.h"
 #include "net/base/network_change_notifier.h"
 
@@ -19,7 +20,10 @@ namespace gcm {
 
 ConnectionEventTracker::ConnectionEventTracker() = default;
 
-ConnectionEventTracker::~ConnectionEventTracker() = default;
+ConnectionEventTracker::~ConnectionEventTracker() {
+  UMA_HISTOGRAM_ENUMERATION("GCM.PendingConnectionEventsAtShutdown",
+                            completed_events_.size(), kMaxClientEvents + 1);
+}
 
 void ConnectionEventTracker::StartConnectionAttempt() {
   // TODO(harkness): Can we dcheck here that there is not an in progress

tools/metrics/histograms/histograms.xml

@@ -19458,6 +19458,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="GCM.PendingConnectionEventsAtShutdown" units="events">
+  <owner>harkness@chromium.org</owner>
+  <summary>
+    Number of connection events which have not been sent to GCM. This is
+    recorded only at the time of shutdown to capture the events which are being
+    lost due to shutdown. Because this is recorded at shutdown, this is not
+    guaranteed to be complete and should be treated as a best effort record.
+  </summary>
+</histogram>
+
 <histogram name="GCM.ReceivedDataMessageBurstSize" units="messages">
   <owner>juyik@chromium.org</owner>
   <summary>Number of messages in a received GCM data message burst.</summary>