Fixed race condition with two calls to port.once

The content script checks twice for which trackers are blocked, since the extension might not yet know about all blocked trackers on the first check (but we want the first check to happen as quickly as possible). These two checks sometimes interleave and cause an error (#162). So don't do the second check until the first has completed.
window.onload seems to work sometimes, but doesn't always trigger for some reason. I'll try the latter approach, sounds like it would work. (Need to sign off soon so may not get back to it until next week.)