Add metrics for the Clear-Site-Data header.

content/browser/browsing_data/clear_site_data_throttle.h

 // Copyright 2016 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.
 
 #ifndef CONTENT_BROWSER_BROWSING_DATA_CLEAR_SITE_DATA_THROTTLE_H_
 #define CONTENT_BROWSER_BROWSING_DATA_CLEAR_SITE_DATA_THROTTLE_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 58 matching lines @@
   void TaskFinished();
 
   // Cached console messages to be output when the RenderFrameHost is ready.
   std::vector<ConsoleMessage> messages_;
   GURL current_url_;
 
   // Whether we are currently waiting for a callback that data clearing has
   // been completed;
   bool clearing_in_progress_;
 
+  // The time when the last clearing operation started. Used when clearing
+  // finishes to compute the duration.
+  base::TimeTicks clearing_started_;
+
   // Needed for asynchronous parsing and deletion tasks.
   base::WeakPtrFactory<ClearSiteDataThrottle> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ClearSiteDataThrottle);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_BROWSING_DATA_CLEAR_SITE_DATA_THROTTLE_H_