Track the ratio of received GCM messages for registered applications

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 6fe2ad8a1e452e18ba9bf68cf15bf17d8b402388..a83aea9b02d928e0568f6462b20d668f2e78aa3f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -17684,6 +17684,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="GCM.DataMessageReceivedHasRegisteredApp"
+    enum="BooleanRegistered">
+  <owner>peter@chromium.org</owner>
+  <summary>
+    Records whether a the registration associated to a received DATA_MESSAGE

[johnme, 2016/09/26 13:29:13]

How about: "Records whether a corresponding registration was found for each
received DATA_MESSAGE or DELETED_MESSAGES message from Google Cloud Messaging."

In any case, fix the "a the" :)

[Peter Beverloo, 2016/09/26 13:31:42]

Done.

+    message, or any DELETED_MESSAGES message from Google Cloud Messaging could
+    be found while processing the received message.
+  </summary>
+</histogram>
+
 <histogram name="GCM.FirstReceivedDataMessageLatencyAfterConnection" units="ms">
   <owner>juyik@chromium.org</owner>
   <summary>