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

google_apis/gaia/account_tracker.cc

Index: google_apis/gaia/account_tracker.cc
diff --git a/google_apis/gaia/account_tracker.cc b/google_apis/gaia/account_tracker.cc
index f424863c9e8c6f9ab1d18aa73d02ac5769a2f508..d40cd20a59b0bd5d75162177953e7342c468c7a7 100644
--- a/google_apis/gaia/account_tracker.cc
+++ b/google_apis/gaia/account_tracker.cc
@@ -177,7 +177,7 @@ void AccountTracker::NotifySignInChanged(const AccountState& account) {
                     OnAccountSignInChanged(account.ids, account.is_signed_in));
 }
 
-void AccountTracker::UpdateSignInState(const std::string account_key,
+void AccountTracker::UpdateSignInState(const std::string& account_key,
                                        bool is_signed_in) {
   // TODO(robliao): Remove ScopedTracker below once https://crbug.com/422460 is
   // fixed.
@@ -198,7 +198,7 @@ void AccountTracker::UpdateSignInState(const std::string account_key,
     NotifySignInChanged(account);
 }
 
-void AccountTracker::StartTrackingAccount(const std::string account_key) {
+void AccountTracker::StartTrackingAccount(const std::string& account_key) {
   // TODO(robliao): Remove ScopedTracker below once https://crbug.com/422460 is
   // fixed.
   tracked_objects::ScopedTracker tracking_profile(
@@ -215,7 +215,7 @@ void AccountTracker::StartTrackingAccount(const std::string account_key) {
   }
 }
 
-void AccountTracker::StopTrackingAccount(const std::string account_key) {
+void AccountTracker::StopTrackingAccount(const std::string& account_key) {
   DVLOG(1) << "StopTracking " << account_key;
   if (ContainsKey(accounts_, account_key)) {
     AccountState& account = accounts_[account_key];
@@ -235,7 +235,7 @@ void AccountTracker::StopTrackingAllAccounts() {
     StopTrackingAccount(accounts_.begin()->first);
 }
 
-void AccountTracker::StartFetchingUserInfo(const std::string account_key) {
+void AccountTracker::StartFetchingUserInfo(const std::string& account_key) {
   // TODO(robliao): Remove ScopedTracker below once https://crbug.com/422460 is
   // fixed.
   tracked_objects::ScopedTracker tracking_profile(