Using microtask checkpoints for resetting the slow script dialog timer

Right now we use the ScriptEvaluated call to reset the "time we have 
been running script" state.  This requires us to have hacks for modal 
state in there, and still has problems if we bounce in and out of 
scripts (like event listeners, say) each of which is quick but which 
cumulatively take lots of time.

Would it make any sense to reset the timer off a microtask checkpoint 
instead?  So conceptually we'd do it like so:

1)  A call to the operation callback records the current time if we're
     in the "reset" state.
2)  Following calls compare the "now" time to the recorded time and put
     up the slow script dialog as needed.
3)  Clicking "stop script" disables script on the page (so if this
     happens inside an event listener, that will not run any more script
     for that event dispatch).
4)  A microtask checkpoint resets our state and reenables script if it
     was disabled due to item 3.  Probably after doing the mutation
     handlers and whatnot.

Thoughts?

-Boris