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..c0b65ff48f52937963527c9b3c21f611d03132ee 100644
--- a/google_apis/gaia/account_tracker.h
+++ b/google_apis/gaia/account_tracker.h
@@ -97,12 +97,17 @@ 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 StartTrackingAccount(const std::string& account_key);
+
+  // Note: |account_key| is passed by value here, because the original
+  // object may be stored in |accounts_| and if so, it will be destroyed
+  // after erasing the key from the map.
   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.