Add rappor metrics for messages received

tools/metrics/rappor/rappor.xml

 <!--
 Copyright 2015 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.
 -->
 
 <rappor-configuration>
 <!--
 This file is used to generate a comprehensive list of Chrome rappor metrics
 along with a detailed description for each histogram. See the design doc at
@@ skipping 240 matching lines @@
 <rappor-metric name="Plugins.SilverlightOriginUrl" type="ETLD_PLUS_ONE">
   <owner>wfh@chromium.org</owner>
   <summary>
     The domain and registry of the top level URL of a page which attempts to
     launch a Silverlight NPAPI plugin, if the client has Silverlight installed
     and enabled. Recorded when the plugin frame appears for each Silverlight
     object found on the page, even if the plugin is click-to-play.
   </summary>
 </rappor-metric>
 
+<rappor-metric name="PushMessaging.MessageReceived.Origin" type="ETLD_PLUS_ONE">
+  <owner>miguelg@chromium.org</owner>
+  <summary>
+    Push Messages received per Origin.
+  </summary>
+</rappor-metric>
+
 <rappor-metric name="Search.DefaultSearchProvider" type="ETLD_PLUS_ONE">
   <owner>holte@chromium.org</owner>
   <summary>
     The domain and registry of the URL for the default search engine.
     Recorded when a profile is opened or after a profile reset.
   </summary>
 </rappor-metric>
 
 <rappor-metric name="ServiceWorker.ControlledPageUrl" type="ETLD_PLUS_ONE">
   <owner>horo@chromium.org</owner>
@@ skipping 56 matching lines @@
   <owner>nparker@chromium.org</owner>
   <summary>
     The domain+registry of a URL that triggered an SSL interstitial.     Domains
     for bad-clock warnings are not reported.
   </summary>
 </rappor-metric>
 
 </rappor-metrics>
 
 </rappor-configuration>