This should silence a relatively high-frequency intermittent that seems to be
specific to Windows-with-WebRender, which fails with "max difference: 168,
number of differing pixels: 30", where the differing pixels are due to a subtle
positioning difference in the dropdown arrow on a select element (not
particularly interesting/worrisome).
Differential Revision: https://phabricator.services.mozilla.com/D119830
