Add RAPPOR metrics of ServiceWorker.ControlledPage

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 127 matching lines @@
 </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>
+  <summary>
+    The domain and registry of the URL of the page which is controlled by a
+    ServiceWorker. Recorded when the page is loaded.
+  </summary>
+</rappor-metric>
+
 <rappor-metric name="Settings.FirstStartupPage" type="ETLD_PLUS_ONE">
   <owner>holte@chromium.org</owner>
   <summary>
     The domain and registry of the URL of the first page opened on startup.
     Recorded when a profile is loaded if preferences specify a fixed set of URLs
     to open on startup.
   </summary>
 </rappor-metric>
 
 <rappor-metric name="Settings.HomePage2" type="ETLD_PLUS_ONE">
@@ skipping 40 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>