Add rappor metrics to track Media Session API usage

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.
 -->
 
 <!--
 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
 http://www.chromium.org/developers/design-documents/rappor
@@ skipping 1298 matching lines @@
 </rappor-metric>
 
 <rappor-metric name="Media.OriginUrl.SRC.VideoCodec.vp9" type="ETLD_PLUS_ONE">
   <owner>xhwang@chromium.org</owner>
   <summary>
     The domain and registry of the URL that loads an HTML5 media player using
     SRC with VP9 video codec.
   </summary>
 </rappor-metric>
 
+<rappor-metric name="Media.Session.APIUsage.Origin" type="ETLD_PLUS_ONE">
+  <owner>avayvod@chromium.org</owner>
+  <owner>mlamouri@chromium.org</owner>
+  <owner>zqzhang@chromium.org</owner>
+  <summary>
+    The eTLD+1 of the URL of frames using the Media Session API.
+  </summary>
+</rappor-metric>
+
 <rappor-metric name="NTP.ExplicitUserAction.PageNavigation.NTPTileClick"
     type="UMA_RAPPOR_TYPE">
   <owner>knn@chromium.org</owner>
   <summary>
     The eTLD+1 of the website visited by a user by clicking on the tile in the
     NewTabPage.
   </summary>
 </rappor-metric>
 
 <rappor-metric name="NTP.ExplicitUserAction.PageNavigation.OmniboxNonSearch"
@@ skipping 1042 matching lines @@
     <flag bit="1" label="IS_REPEAT_VISIT"/>
     <summary>
       Bitfield of the state from an SSL warning interstitial.
     </summary>
   </flags-field>
 </rappor-metric>
 
 </rappor-metrics>
 
 </rappor-configuration>