Combine some access to the thread local variable for script thread. (#38752)

This combines some access to the thread local variable for script
thread.

- We introduce a new UserInteractingScriptGuard which on drop handles
  the resetting of was_interacting to the previous value. Sometimes
throughout the code `ScriptThread::is_user_interacting` was reset to the
previous value while sometimes just set to false. This should
remove this footgun.
- This also reduces the amount of thread local access for
MutationObservers and task queue.

Testing: WPT tests should cover this.
Fixes: This addresses part of
https://github.com/servo/servo/issues/37969 but there is probably still
stuff to be done.

---------

Signed-off-by: Narfinger <Narfinger@users.noreply.github.com>
Signed-off-by: Josh Matthews <josh@joshmatthews.net>
Co-authored-by: Josh Matthews <josh@joshmatthews.net>

components/script/timers.rs

@@ -589,8 +589,7 @@ impl JsTimerTask {
         // prep for step ? in nested set_timeout_or_interval calls
         timers.nesting_level.set(self.nesting_level);
 
-        let was_user_interacting = ScriptThread::is_user_interacting();
-        ScriptThread::set_user_interacting(self.is_user_interacting);
+        let _guard = ScriptThread::user_interacting_guard();
         match self.callback {
             InternalTimerCallback::StringTimerCallback(ref code_str) => {
                 // Step 6.4. Let settings object be global's relevant settings object.
@@ -630,7 +629,6 @@ impl JsTimerTask {
                 let _ = function.Call_(this, arguments, value.handle_mut(), Report, can_gc);
             },
         };
-        ScriptThread::set_user_interacting(was_user_interacting);
 
         // reset nesting level (see above)
         timers.nesting_level.set(0);