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

ios/web/public/test/web_view_interaction_test_util.mm

Index: ios/web/public/test/web_view_interaction_test_util.mm
diff --git a/ios/web/public/test/web_view_interaction_test_util.mm b/ios/web/public/test/web_view_interaction_test_util.mm
index f29ee56d0102500a165cd8c4a1a6b08d7ba2b762..da4e786561c629b4084dad239cb5510a9b337e37 100644
--- a/ios/web/public/test/web_view_interaction_test_util.mm
+++ b/ios/web/public/test/web_view_interaction_test_util.mm
@@ -52,6 +52,7 @@ std::unique_ptr<base::Value> ExecuteJavaScript(web::WebState* web_state,
   // Fixes the following compilation failure:
   //   ../web_view_matchers.mm:ll:cc: error: call to implicitly-deleted copy
   //       constructor of 'std::unique_ptr<base::Value>'
+  // TODO(crbug.com/703565): remove std::move() once Xcode 9.0+ is required.
   std::unique_ptr<base::Value> stack_result = std::move(result);
   return stack_result;
 }