This PR fixes yet another case in the windows startup test that violates the render rule, which caused https://github.com/flutter/engine/pull/45300 to be reverted.

Although the `FlutterView.render` call is within `onBeginFrame`, there is an `await` before the call, causing the call to fall out of the synchronous scope.

I've added this problem to the documentation of `FlutterView.render` in https://github.com/flutter/engine/pull/45555.