Warn if other tabs under control of your SW are open

[paulirish]

Following up from #598

If there are other tabs open, then we're unable to remove a serviceworker from play. It'll come back to life.

We'd like to warn the user to close any additional tabs that may be under control of the worker. We can use:

• tabs API in the extension
• List method for CLI

And we'll be looking for other tabs that may be under control of the SW. We could use the registration's scopeURL and/or the site's origin and look for potential matches.

[paulirish]

the workerVersionUpdated event (i think) returns an array of clients for the SW. We can see if there are more clients connected.

[wardpeet]

I'm up for this one :)