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/servoparser/xml.rs

@@ -35,6 +35,7 @@ impl Tokenizer {
             current_line: Cell::new(1),
             script: Default::default(),
             parsing_algorithm: ParsingAlgorithm::Normal,
+            custom_element_reaction_stack: document.custom_element_reaction_stack(),
         };
 
         let tb = XmlTreeBuilder::new(sink, Default::default());