Provide an easier way to listen for waiting/activated/redundant Service Workers

Breaking off from #1222.

ServiceWorker.state has five states: installing, installed (waiting), activating, activated, and redundant.

Today, client-side code can listen for installing SWs with ServiceWorkerRegistration.onupdatefound; activating can be tracked with navigator.serviceWorker.oncontrollerchange.

The other three states are annoying to listen for.

function listenForStateChanges(reg, callback) {
  if (reg.installing) reg.installing.addEventListener('statechange', callback);
  if (reg.waiting) reg.waiting.addEventListener('statechange', callback);
  reg.addEventListener('updatefound', function() {
    reg.installing.addEventListener('statechange', callback);
  });
}

That's too bad, because lots of users want to track the installed state so they can show a refresh banner. Furthermore, some users apparently want to track redundant SWs for error handling. And it turns out that refreshing the page as the page is activating is slightly too soon; you should wait for the activated event, to avoid a minor performance bug.

In #1222 we observed that awaiting these states would be way easier if statechange bubbled up to the registration, so users who wanted to track the redundant and activated states would just do this:

reg.addEventListener('statechange', callback);

Start with the ServiceWorker.state states and the ServiceWorkerRegistration event behavior described here, then review the related discussion in #1222. Compare the existing updatefound and controllerchange paths with the proposed registration-level statechange listener; done should make waiting, activated, and redundant transitions observable through the registration.