[Sync] Add tab sync latency UMA

chrome/browser/ui/webui/foreign_session_handler.h

 // Copyright 2012 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 CHROME_BROWSER_UI_WEBUI_FOREIGN_SESSION_HANDLER_H_
 #define CHROME_BROWSER_UI_WEBUI_FOREIGN_SESSION_HANDLER_H_
 
 #include <vector>
 
 #include "base/time/time.h"
@@ skipping 76 matching lines @@
 
   void HandleSetForeignSessionCollapsed(const base::ListValue* args);
 
   // Helper method to create JSON compatible objects from Session objects.
   bool SessionWindowToValue(const ::sessions::SessionWindow& window,
                             base::DictionaryValue* dictionary);
 
   // The Registrar used to register ForeignSessionHandler for notifications.
   content::NotificationRegistrar registrar_;
 
+  // The time at which this WebUI was created. Used to calculate how long
+  // the WebUI was present before the sessions data was visible.
+  base::TimeTicks load_attempt_time_;
+
   DISALLOW_COPY_AND_ASSIGN(ForeignSessionHandler);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_UI_WEBUI_FOREIGN_SESSION_HANDLER_H_