Report: top-level warn if initialUrl and finalUrl are different

[ebidel]

We need to figure out some way of notifying the user if the finalUrl ended up on a different origin than the initialUrl. Mainly for the CLI and WPT cases where users can put in a URL. For the CRX / DT, users typically end up on the final URL before testing.

Separating this out from #715. @brendankenny said:

Sounds good. To close the loop: another issue with initial vs final URL is clearing storage, which is done per origin. If a redirect goes cross origin the cache, SW, etc are not cleared.

In the CLI this is only a problem if not using the default clean Chrome. In the extension you're usually already on the final (possibly redirected) URL, except when sites redirect you to a different origin due to mobile emulation.

[brendankenny]

HTTP -> HTTPS redirects should get this warning too (#2397)

[connorjclark]

I'm not sure we need to have different warnings if just the origin changes. Let's do just a generic warning if requested !== final.