script: Reduce ScriptThread TLS usage (#38875)

We store a pointer to the ScriptThread singleton for a thread in
thread-local storage. While we don't have yet have profiles pointing to
this TLS access as a hot spot, we can remove a potential performance
footgun without a lot of effort by passing around small pieces of data
that we otherwise need to fetch from the ScriptThread.

Testing: Existing WPT is sufficient
Fixes: part of #37969

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script/dom/xmlhttprequest.rs

@@ -1535,6 +1535,7 @@ impl XMLHttpRequest {
             false,
             Some(doc.insecure_requests_policy()),
             doc.has_trustworthy_ancestor_origin(),
+            doc.custom_element_reaction_stack(),
             can_gc,
         )
     }