If two loading documents hit the sheet cache and we coalesce the
resource load, there's nothing that prevents the load event on the
second document from firing right now, and there should be.
While at it, also fix the handling of the pending load count, though
it has no correctness impact on the particular test we're fixing here...
We were never decrementing it, which is of course wrong. However it
kinda ended up working because it just causes us to not defer more
loads.
The new assertions and responsibility of the counter should ensure it
stays correct.
Differential Revision: https://phabricator.services.mozilla.com/D80583
3dba84f650