Before this change, when trying to load a JSONP script that calls the JSONP callback inside a
microtask, it will fail in Internet Explorer 11 and EdgeHTML. This commit changes the onLoad cleanup
to be queued after the loaded endpoint executed any potential microtask itself. This ensures that
the aforementioned browsers will first evaluate the loaded script calling the JSONP callback and
only then run the cleanup inside onLoad.
Fixes#39496
PR Close#39512
39266654e6