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The existing `screenIfViewLoaded` and `windowSceneIfLoaded` functions are private helpers of `FlutterViewController` class. This PR moves the logic to a category of the `UIViewController`, so it can be reused for any `UIViewController`s. *List which issues are fixed by this PR. You must list at least one issue.* https://github.com/flutter/engine/pull/43972#discussion_r1282214557 *If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].* [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style