Replace more ObserverList with base::ObserverList.

chromeos/dbus/update_engine_client.cc

Index: chromeos/dbus/update_engine_client.cc
diff --git a/chromeos/dbus/update_engine_client.cc b/chromeos/dbus/update_engine_client.cc
index 31b9366cfbc67a571b5bcf8b62cadbf5b90f2b4a..7a0bb4c6283e33b4e0c4b0dc8de28154c7e74cf5 100644
--- a/chromeos/dbus/update_engine_client.cc
+++ b/chromeos/dbus/update_engine_client.cc
@@ -383,7 +383,7 @@ class UpdateEngineClientImpl : public UpdateEngineClient {
   }
 
   dbus::ObjectProxy* update_engine_proxy_;
-  ObserverList<Observer> observers_;
+  base::ObserverList<Observer> observers_;
   Status last_status_;
 
   // Note: This should remain the last member so it'll be destroyed and
@@ -525,7 +525,7 @@ class UpdateEngineClientFakeImpl : public UpdateEngineClientStubImpl {
     }
   }
 
-  ObserverList<Observer> observers_;
+  base::ObserverList<Observer> observers_;
   Status last_status_;
 
   base::WeakPtrFactory<UpdateEngineClientFakeImpl> weak_factory_;