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

ios/chrome/browser/signin/signin_manager_factory.cc

Index: ios/chrome/browser/signin/signin_manager_factory.cc
diff --git a/ios/chrome/browser/signin/signin_manager_factory.cc b/ios/chrome/browser/signin/signin_manager_factory.cc
index 201daba977b654557a9615f4d7f5be70565ebdcc..aa637b9f25b0877e9876bbe97635833028f598d6 100644
--- a/ios/chrome/browser/signin/signin_manager_factory.cc
+++ b/ios/chrome/browser/signin/signin_manager_factory.cc
@@ -93,6 +93,7 @@ std::unique_ptr<KeyedService> SigninManagerFactory::BuildServiceInstanceFor(
   service->Initialize(GetApplicationContext()->GetLocalState());
   for (auto& observer : observer_list_)
     observer.SigninManagerCreated(service.get());
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(service);
 }