Add a new way to record Rappor metrics for MIDI permissions.

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 724 matching lines @@
 
 <rappor-metric name="ContentSettings.PermissionRequested.Geolocation.Url2"
     type="LOW_FREQUENCY_ETLD_PLUS_ONE">
   <owner>dominickn@chromium.org</owner>
   <owner>miguelg@chromium.org</owner>
   <summary>
     The domain that issues a Geolocation permission prompt.
   </summary>
 </rappor-metric>
 
+<rappor-metric name="ContentSettings.PermissionRequested.Midi.Url"
+    type="ETLD_PLUS_ONE">
+  <owner>toyoshim@chromium.org</owner>
+  <summary>
+    **DEPRECATED: replaced by ContentSettings.PermissionRequested.Midi.Url2
+    as of M53. The domain that requests using MIDI devices.
+  </summary>
+</rappor-metric>
+
+<rappor-metric name="ContentSettings.PermissionRequested.Midi.Url2"
+    type="LOW_FREQUENCY_ETLD_PLUS_ONE">
+  <owner>toyoshim@chromium.org</owner>
+  <summary>
+    The domain that requests using MIDI devices.
+  </summary>
+</rappor-metric>
+
 <rappor-metric name="ContentSettings.PermissionRequested.Notifications.Url"
     type="ETLD_PLUS_ONE">
   <owner>miguelg@chromium.org</owner>
   <summary>
     **DEPRECATED: replaced by
     ContentSettings.PermissionRequested.Notifications.Url2 as of M52. The
     domain that issues a Notification permission prompt.
   </summary>
 </rappor-metric>
 
@@ skipping 1129 matching lines @@
     <flag>Bit 1: IS_REPEAT_VISIT</flag>
     <summary>
       Bitfield of the state from an SSL warning interstitial.
     </summary>
   </flags-field>
 </rappor-metric>
 
 </rappor-metrics>
 
 </rappor-configuration>