Try using window loading events to trigger vectorization instead of tabs.onUpdated

[danielhertenstein]

Currently, the vectorizer (though visit.js) listens for the tabs.onUpdated event to trigger vectorization of each tab. Perhaps we could instead use window.onload or window.addEventListener('load', ...). We will need to ensure that the occurrence of these events does indeed mean that our samples have finished loading everything we expect them to load. We should remember that we are not loading any Javascript in our frozen samples. Making this change may require some message passing between visit.js, background.js, and contentScript.js.

[erikrose]

The motivation behind this was speed: perhaps we would no longer need the unconditional delay of (by default) 5 seconds. And perhaps we wouldn't have the problem of failures when the delay is set too low.