Cleanup: Pass std::string as const reference from google_apis/ and components/gcm_driver/

google_apis/gaia/account_tracker.h

Index: google_apis/gaia/account_tracker.h
diff --git a/google_apis/gaia/account_tracker.h b/google_apis/gaia/account_tracker.h
index 61d3748868bf593a70eebdf17f3c8ada3c0c55ab..653f7b7491ddec4d1c8f0e7d1f00e5347e23e175 100644
--- a/google_apis/gaia/account_tracker.h
+++ b/google_apis/gaia/account_tracker.h
@@ -97,12 +97,12 @@ class AccountTracker : public OAuth2TokenService::Observer,
   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.