Replace more ObserverList with base::ObserverList.

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 3ae0d869c897ecbbccfa487a4f88be71ff093388..846eb5087772d5ef8da11df04e6c10c3cb6ed8b3 100644
--- a/extensions/browser/extension_host.h
+++ b/extensions/browser/extension_host.h
@@ -209,8 +209,8 @@ class ExtensionHost : public DeferredStartRenderHost,
   // started only once the ExtensionHost has exited the ExtensionHostQueue.
   scoped_ptr<base::ElapsedTimer> load_start_;
 
-  ObserverList<ExtensionHostObserver> observer_list_;
-  ObserverList<DeferredStartRenderHostObserver>
+  base::ObserverList<ExtensionHostObserver> observer_list_;
+  base::ObserverList<DeferredStartRenderHostObserver>
       deferred_start_render_host_observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionHost);