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

components/signin/core/browser/account_fetcher_service.cc

 // Copyright 2015 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.
 
 #include "components/signin/core/browser/account_fetcher_service.h"
 
 #include "base/command_line.h"
 #include "base/metrics/field_trial.h"
 #include "base/prefs/pref_service.h"
 #include "base/profiler/scoped_tracker.h"
@@ skipping 87 matching lines @@
   shutdown_called_ = true;
 }
 
 void AccountFetcherService::EnableNetworkFetches() {
   DCHECK(CalledOnValidThread());
   DCHECK(!network_fetches_enabled_);
   network_fetches_enabled_ = true;
   // If there are accounts in |pending_user_info_fetches_|, they were deemed
   // invalid after being loaded from prefs and need to be fetched now instead of
   // waiting after the timer.
-  for (std::string account_id : pending_user_info_fetches_)
+  for (const std::string& account_id : pending_user_info_fetches_)
     StartFetchingUserInfo(account_id);
   pending_user_info_fetches_.clear();
 
   // Now that network fetches are enabled, schedule the next refresh.
   ScheduleNextRefresh();
 }
 
 bool AccountFetcherService::IsAllUserInfoFetched() const {
   return user_info_requests_.empty();
 }
@@ skipping 194 matching lines @@
   UpdateChildInfo();
   account_tracker_service_->StopTrackingAccount(account_id);
 }
 
 void AccountFetcherService::OnRefreshTokensLoaded() {
   // OnRefreshTokenAvailable has been called for all accounts by this point.
   // Maybe remove this after further investigation.
   RefreshAllAccountInfo(true);
   UpdateChildInfo();
 }