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

components/signin/core/browser/account_fetcher_service.cc

Index: components/signin/core/browser/account_fetcher_service.cc
diff --git a/components/signin/core/browser/account_fetcher_service.cc b/components/signin/core/browser/account_fetcher_service.cc
index 139ad28b4dccbfbc995725ae2f2ad7731bcc1275..22c804f6b496723a0a935e75abe82b37765e7d51 100644
--- a/components/signin/core/browser/account_fetcher_service.cc
+++ b/components/signin/core/browser/account_fetcher_service.cc
@@ -105,7 +105,7 @@ void AccountFetcherService::EnableNetworkFetches() {
   // 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();