Content Settings: Add RevocationObserver to measure when permissions are revoked

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.
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 <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 @@
     recorded as a multi-dimensional metric in Permissions.Action.AudioCapture.
   </summary>
 </rappor-metric>
 
 <rappor-metric
     name="ContentSettings.PermissionActions_AudioCapture.Revoked.Url"
     type="ETLD_PLUS_ONE">
   <owner>tsergeant@chromium.org</owner>
   <summary>
     The domain for which an AudioCapture permission was revoked. Also recorded
-    as a multi-dimensional metric in Permissions.Action.AudioCapture.
+    as a multi-dimensional metric in Permissions.Action.AudioCapture. Note: This
+    metric is recorded for more revocation actions as of 2016-04-01 (M51).
   </summary>
 </rappor-metric>
 
 <rappor-metric name="ContentSettings.PermissionActions_Camera.Revoked.Url"
     type="ETLD_PLUS_ONE">
   <owner>jialiul@chromium.org</owner>
   <summary>
     **DEPRECATED. Renamed to
     ContentSettings.PermissionActions_VideoCapture.Revoked.Url on
     2016-01-07.** The domain for which a camera permission was revoked.
@@ skipping 30 matching lines @@
   <owner>miguelg@chromium.org</owner>
   <summary>
     The domain for which a Geolocation permission prompt was Ignored.
   </summary>
 </rappor-metric>
 
 <rappor-metric name="ContentSettings.PermissionActions_Geolocation.Revoked.Url"
     type="ETLD_PLUS_ONE">
   <owner>jialiul@chromium.org</owner>
   <summary>
-    The domain for which a Geolocation permission was revoked.
+    The domain for which a Geolocation permission was revoked. Note: This
+    metric is recorded for more revocation actions as of 2016-04-01 (M51).
   </summary>
 </rappor-metric>
 
 <rappor-metric name="ContentSettings.PermissionActions_Mic.Revoked.Url"
     type="ETLD_PLUS_ONE">
   <owner>jialiul@chromium.org</owner>
   <summary>
     **DEPRECATED. Renamed to
     ContentSettings.PermissionActions_AudioCapture.Revoked.Url on
     2016-01-07.** The domain for which a microphone permission was revoked.
@@ skipping 34 matching lines @@
   <summary>
     The domain for which a Notification permission prompt was Ignored.
   </summary>
 </rappor-metric>
 
 <rappor-metric
     name="ContentSettings.PermissionActions_Notifications.Revoked.Url"
     type="ETLD_PLUS_ONE">
   <owner>jialiul@chromium.org</owner>
   <summary>
-    The domain for which a Notification permission was revoked.
+    The domain for which a Notification permission was revoked. Note: This
+    metric is recorded for more revocation actions as of 2016-04-01 (M51).
   </summary>
 </rappor-metric>
 
 <rappor-metric name="ContentSettings.PermissionActions_VideoCapture.Denied.Url"
     type="ETLD_PLUS_ONE">
   <owner>kcarattini@chromium.org</owner>
   <owner>miguelg@chromium.org</owner>
   <owner>tsergeant@chromium.org</owner>
   <summary>
     The domain for which a VideoCapture permission prompt was Denied. Also
@@ skipping 36 matching lines @@
     recorded as a multi-dimensional metric in Permissions.Action.VideoCapture.
   </summary>
 </rappor-metric>
 
 <rappor-metric
     name="ContentSettings.PermissionActions_VideoCapture.Revoked.Url"
     type="ETLD_PLUS_ONE">
   <owner>tsergeant@chromium.org</owner>
   <summary>
     The domain for which an VideoCapture permission was revoked. Also recorded
-    as a multi-dimensional metric in Permissions.Action.VideoCapture.
+    as a multi-dimensional metric in Permissions.Action.VideoCapture. Note: This
+    metric is recorded for more revocation actions as of 2016-04-01 (M51).
   </summary>
 </rappor-metric>
 
 <rappor-metric name="ContentSettings.PermissionRequested.Geolocation.Url"
     type="ETLD_PLUS_ONE">
   <owner>miguelg@chromium.org</owner>
   <summary>
     The domain that issues a Geolocation permission prompt.
   </summary>
 </rappor-metric>
@@ skipping 994 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>