Keep the player window in a similar position to where it was before a resize.
The player window edge closest to a screen edge will be maintained in the X
and Y dimensions.

A video in the bottom right will have its bottom right corner stay in place,
while a video in the top left will have its top left corner stay in place.

Differential Revision: https://phabricator.services.mozilla.com/D52064

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extra : moz-landing-system : lando