Add PendingEvents UMA to ConnectionEventTracker shutdown.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 543fa7e2cc5986675dd7fe8705bbb80713bfbfd2..75a095d0cb51d01b37c5442912fe05673bb52771 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -19102,6 +19102,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="GCM.PendingEvents">

[Peter Beverloo, 2016/12/22 11:24:36]

units="events"

[harkness, 2016/12/23 13:52:56]

Done.

+  <owner>harkness@chromium.org</owner>
+  <summary>
+    Number of connection events which have not been sent to the GCM. This is

[Peter Beverloo, 2016/12/22 11:24:36]

nit: s/the//

[harkness, 2016/12/23 13:52:56]

Done.

+    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>