Migrate ArcBridgeService::Observer and ArcSession::Observer.

chrome/browser/chromeos/arc/print/arc_print_service.h

 // Copyright 2016 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 CHROME_BROWSER_CHROMEOS_ARC_PRINT_ARC_PRINT_SERVICE_H_
 #define CHROME_BROWSER_CHROMEOS_ARC_PRINT_ARC_PRINT_SERVICE_H_
 
 #include "base/macros.h"
 #include "components/arc/arc_service.h"
 #include "components/arc/common/print.mojom.h"
 #include "components/arc/instance_holder.h"
 #include "mojo/public/cpp/bindings/binding.h"
 
 namespace arc {
 
 class ArcBridgeService;
 
 class ArcPrintService : public ArcService,
                         public InstanceHolder<mojom::PrintInstance>::Observer,
                         public mojom::PrintHost {
  public:
   explicit ArcPrintService(ArcBridgeService* bridge_service);
   ~ArcPrintService() override;
 
-  // ArcBridgeService::Observer overrides.
+  // InstanceHolder<mojom::PrintInstance>::Observer:
   void OnInstanceReady() override;
 
   void Print(mojo::ScopedHandle pdf_data) override;
 
  private:
   mojo::Binding<mojom::PrintHost> binding_;
 
   DISALLOW_COPY_AND_ASSIGN(ArcPrintService);
 };
 
 }  // namespace arc
 
 #endif  // CHROME_BROWSER_CHROMEOS_ARC_PRINT_ARC_PRINT_SERVICE_H_