Much like everything else. This is important because the front-end uses
sizemode for styling.

The comment I'm removing doesn't really make much sense, since sizemode
and size are persisted separately (using different dirty flags) so it's
not like we're saving duplicate work or something.

The delay this fixes existed already on Linux since ~forever at least,
haven't tested macOS.

Depends on D159578

Differential Revision: https://phabricator.services.mozilla.com/D159579