To avoid trimming pixels at the top / left.
This makes it closer to non-WR[1], and fixes both the checkboxes getting
cut off and the master password field.
[1]: non-WR at least at 124 scaling on a hiDPI display is still perfect, though I saw nin symmetric borders at other resolutions, so we might be able to improve here further.
Differential Revision: https://phabricator.services.mozilla.com/D7251
--HG--
extra : moz-landing-system : lando