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

extensions/browser/extension_host.h

Index: extensions/browser/extension_host.h
diff --git a/extensions/browser/extension_host.h b/extensions/browser/extension_host.h
index 567499ba06ff4a3c8c3cca1eeb420eba9159eb41..cd147107164b03e9cfa3b8b20c14f5ddb76718ba 100644
--- a/extensions/browser/extension_host.h
+++ b/extensions/browser/extension_host.h
@@ -197,8 +197,8 @@ class ExtensionHost : public content::WebContentsDelegate,
   // 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_;
+  // Used to measure how long it takes to load the initial URL.
+  scoped_ptr<base::ElapsedTimer> since_created_;
 
   ObserverList<ExtensionHostObserver> observer_list_;