Cleanup: Pass std::string as const reference if possible

google_apis/gaia/account_tracker.h

 // Copyright 2014 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 GOOGLE_APIS_GAIA_ACCOUNT_TRACKER_H_
 #define GOOGLE_APIS_GAIA_ACCOUNT_TRACKER_H_
 
 #include <map>
 #include <string>
 #include <vector>
@@ skipping 79 matching lines @@
  private:
   struct AccountState {
     AccountIds ids;
     bool is_signed_in;
   };
 
   void NotifyAccountAdded(const AccountState& account);
   void NotifyAccountRemoved(const AccountState& account);
   void NotifySignInChanged(const AccountState& account);
 
-  void UpdateSignInState(const std::string account_key, bool is_signed_in);
+  void UpdateSignInState(const std::string& account_key, bool is_signed_in);
 
-  void StartTrackingAccount(const std::string account_key);
-  void StopTrackingAccount(const std::string account_key);
+  void StartTrackingAccount(const std::string& account_key);
+  void StopTrackingAccount(const std::string& account_key);
   void StopTrackingAllAccounts();
-  void StartFetchingUserInfo(const std::string account_key);
+  void StartFetchingUserInfo(const std::string& account_key);
   void DeleteFetcher(AccountIdFetcher* fetcher);
 
   IdentityProvider* identity_provider_;  // Not owned.
   scoped_refptr<net::URLRequestContextGetter> request_context_getter_;
   std::map<std::string, AccountIdFetcher*> user_info_requests_;
   std::map<std::string, AccountState> accounts_;
   base::ObserverList<Observer> observer_list_;
   bool shutdown_called_;
 };
 
@@ skipping 28 matching lines @@
   AccountTracker* tracker_;
   const std::string account_key_;
 
   scoped_ptr<OAuth2TokenService::Request> login_token_request_;
   scoped_ptr<gaia::GaiaOAuthClient> gaia_oauth_client_;
 };
 
 }  // namespace extensions
 
 #endif  // GOOGLE_APIS_GAIA_ACCOUNT_TRACKER_H_