If a function that captures a GC-allocated object is owned by another
GC-allocated object, it is more preferable to use JS::HeapFunction.
This is because JS::HeapFunction is visited, unlike introducing a new
heap root as JS::SafeFunction does.
The HTMLMediaElement will need to stop fetching processes when its load
algorithm is invoked while a fetch is ongoing. We don't have a way to
really stop the process, due to the way it runs on nested deferred task
invocations. So for now, this swaps the fetch callbacks (e.g. to process
a fetch response) with empty callbacks.
This makes Fetch rely less on using main_thread_vm().current_realm(),
which relies on the dummy execution context if no JavaScript is
currently running.
