For example, running `alert(1)` will pause the event loop, during which
time no JavaScript should execute. This patch extends this disruption to
microtasks. This avoids a crash inside the microtask executor, which
asserts the JS execution context stack is empty.
This makes us behave the same as Firefox in the following page:
<script>
queueMicrotask(() => {
console.log("inside microtask");
});
alert("hi");
</script>
Before the aforementioned assertion was added, we would execute that
microtask before showing the alert. Firefox does not do this, and now
we don't either.
This makes it more convenient to use the 'relvant agent' concept,
instead of the awkward dynamic casts we needed to do for every call
site.
mutation_observers is also changed to hold a GC::Root instead of raw
GC::Ptr. Somehow this was not causing problems before, but trips up CI
after these changes.
Instead of collecting all documents in a big vector and then filtering
the vector (twice!) with remove_all_matching(), we now pass a filter
callback to documents_in_this_event_loop_matching() and avoid all the
extra shuffling work.
Saw this stuff hogging ~20% of CPU time when profiling a WebContent
process in the middle of a WPT run.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
Now that the heap has no knowledge about a JavaScript realm and is
purely for managing the memory of the heap, it does not make sense
to name this function to say that it is a non-realm variant.
