Partially reverting a3149c1ce9
Spinning the event loop was causing a crash on:
https://wpt.live/url/percent-encoding.window.html
As it was turning what is meant to be a synchronous operation into an
asynchronous one.
The sequence demonstrated by the reproducing test is as follows:
* A src attribute is changed for the iframe
* process_the_iframe_attributes entered with valid content navigable
* Event loop is spun, allowing the queued iframe removal to execute
* process_the_iframe_attributes continues with null content navigable
* 💥
