arc: Implement safe access to ArcAuthService.

chrome/browser/chromeos/arc/arc_auth_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_ARC_AUTH_SERVICE_H_
 #define CHROME_BROWSER_CHROMEOS_ARC_ARC_AUTH_SERVICE_H_
 
 #include <memory>
 #include <ostream>
 #include <string>
@@ skipping 135 matching lines @@
   void StartLso();
 
   // Called from Arc support platform app to set auth code and start arc.
   void SetAuthCodeAndStartArc(const std::string& auth_code);
 
   // Called from Arc support platform app when user cancels signing.
   void CancelAuthCode();
 
   bool IsArcManaged() const;
   bool IsArcEnabled() const;
+
+  // This requires Arc to be allowed (|IsAllowed|)for current profile.
   void EnableArc();
   void DisableArc();
 
   // syncable_prefs::PrefServiceSyncableObserver
   void OnIsSyncingChanged() override;
 
   // syncable_prefs::SyncedPrefObserver
   void OnSyncedPrefChanged(const std::string& path, bool from_sync) override;
 
   // ArcAuthContextDelegate:
@@ skipping 53 matching lines @@
   base::WeakPtrFactory<ArcAuthService> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ArcAuthService);
 };
 
 std::ostream& operator<<(std::ostream& os, const ArcAuthService::State& state);
 
 }  // namespace arc
 
 #endif  // CHROME_BROWSER_CHROMEOS_ARC_ARC_AUTH_SERVICE_H_