Bug 1757637 - Use the ImageContainer to determine video{Width,Height} instead...

Bug 1757637 - Use the ImageContainer to determine video{Width,Height} instead of passing through the state machine. r=alwu

This means there is significantly less chance of desynchronization between the
actual size of the image and the reported size of the image.

Differential Revision: https://phabricator.services.mozilla.com/D149150