Replace the ExtensionHost load UMAs (Extensions.BackgroundPageLoadTime and friends)

extensions/browser/extension_host.h

 // Copyright 2014 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 EXTENSIONS_BROWSER_EXTENSION_HOST_H_
 #define EXTENSIONS_BROWSER_EXTENSION_HOST_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 179 matching lines @@
   // Messages sent out to the renderer that have not been acknowledged yet.
   std::set<int> unacked_messages_;
 
   content::NotificationRegistrar registrar_;
 
   ExtensionFunctionDispatcher extension_function_dispatcher_;
 
   // The type of view being hosted.
   ViewType extension_host_type_;
 
-  // Used to measure how long it's been since the host was created.
-  base::ElapsedTimer since_created_;
+  // Measures how long since the initial URL started loading.
+  scoped_ptr<base::ElapsedTimer> load_start_;
 
   ObserverList<ExtensionHostObserver> observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionHost);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_EXTENSION_HOST_H_