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

ios/chrome/browser/autocomplete/in_memory_url_index_factory.cc

Index: ios/chrome/browser/autocomplete/in_memory_url_index_factory.cc
diff --git a/ios/chrome/browser/autocomplete/in_memory_url_index_factory.cc b/ios/chrome/browser/autocomplete/in_memory_url_index_factory.cc
index ed876b253b337bd6661e029c4a73b3e78ff71aaf..9f46f3e9c2762ae701d886fce73cfcc778425326 100644
--- a/ios/chrome/browser/autocomplete/in_memory_url_index_factory.cc
+++ b/ios/chrome/browser/autocomplete/in_memory_url_index_factory.cc
@@ -39,6 +39,7 @@ std::unique_ptr<KeyedService> BuildInMemoryURLIndex(
       web::WebThread::GetBlockingPool(), browser_state->GetStatePath(),
       schemes_to_whilelist));
   in_memory_url_index->Init();
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   return std::move(in_memory_url_index);
 }