Bug 1582502 - Don't interpret a click at 0,0 of a fullscreen video as a click on the Picture-in-Picture toggle. r=JSON_voorhees

We weren't handling the case where the toggle has no dimensions, so its X and Y
coordinate would be at 0,0, which would match a click in the top-left corner of
the screen for a video that was fullscreened.

This adds a check that ensures that the toggle has both width and height before
checking whether or not a mouse event occurred within it.

Differential Revision: https://phabricator.services.mozilla.com/D47789

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extra : moz-landing-system : lando