Migrate ArcBridgeService::Observer and ArcSession::Observer.

components/arc/test/fake_arc_bridge_service.cc

 // Copyright 2015 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.
 
 #include "components/arc/test/fake_arc_bridge_service.h"
 
 namespace arc {
 
 FakeArcBridgeService::FakeArcBridgeService() = default;
 
@@ skipping 16 matching lines @@
 void FakeArcBridgeService::SetReady() {
   if (state() != State::READY)
     SetState(State::READY);
 }
 
 void FakeArcBridgeService::SetStopped() {
   if (state() != State::STOPPED)
     SetState(State::STOPPED);
 }
 
-bool FakeArcBridgeService::HasObserver(const Observer* observer) {
+bool FakeArcBridgeService::HasObserver(const ArcSessionObserver* observer) {
   return observer_list().HasObserver(observer);
 }
 
 }  // namespace arc