With this change, we now have ~1200 CellAllocators across both LibJS and
LibWeb in a normal WebContent instance.
This gives us a minimum heap size of 4.7 MiB in the scenario where we
only have one cell allocated per type. Of course, in practice there will
be many more of each type, so the effective overhead is quite a bit
smaller than that in practice.
I left a few types unconverted to this mechanism because I got tired of
doing this. :^)
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.
