rMOZILLABETA5a63ac24e8f7

Bug 1568382 - wait for PiP window focus before resolving triggerPictureInPicture. r=mconley

`toolkit/components/pictureinpicture/tests/browser_fullscreen.js` is
no longer intermittently failing to `requestFullscreen()` after this
change. The test was creating the PiP window with
`triggerPictureInPicture` and then immediately moving focus to the
original window where fullscreen is requested. It's possible that
the browser had not given focus to the PiP window yet and would do
so after the test had changed…