[ios] Add TODO to record usages of "return std::move(foo)" pattern.

ios/chrome/browser/signin/account_reconcilor_factory.cc

Index: ios/chrome/browser/signin/account_reconcilor_factory.cc
diff --git a/ios/chrome/browser/signin/account_reconcilor_factory.cc b/ios/chrome/browser/signin/account_reconcilor_factory.cc
index a4b81e3438fbbfac731db97aea94e07a9b202590..8b84a8c9fb277f458d313046da739c5b94816819 100644
--- a/ios/chrome/browser/signin/account_reconcilor_factory.cc
+++ b/ios/chrome/browser/signin/account_reconcilor_factory.cc
@@ -52,6 +52,7 @@ std::unique_ptr<KeyedService> AccountReconcilorFactory::BuildServiceInstanceFor(
       GaiaCookieManagerServiceFactory::GetForBrowserState(
           chrome_browser_state)));
   reconcilor->Initialize(true /* start_reconcile_if_tokens_available */);
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(reconcilor);
 }