Improve interaction of css zoom and non-absolute CSS units.
The values that we take from our parent should be zoomed in. Similarly,
root values should also be zoomed in by the effective zoom (for that, we
unzoom root values to zoom-independent pixels when storing them on the
device).
Container-relative units probably need more care / thought, because they
are in the layout (so zoom-independent) coordinate space already, since
they come from frames. Bug 1894104 is filed for this.
Differential Revision: https://phabricator.services.mozilla.com/D208599
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1892676
gecko-commit: 5b852a31be5749f41b4b7389efee811b90f5c03e
gecko-reviewers: dholbert