Keep using the static position as we did before. As long as we don't
have anchor positioning this seems better than whack-a-mole-ing every
possible urlbar position change.
In order to make the fullscreen animation work, make our position
computation ignore transforms, which is how anchor positioning would
behave anyways.
Differential Revision: https://phabricator.services.mozilla.com/D227758
a1ae05d74e