…e size. r=gw

(cherry picked from commit 348b45266ac2c2c65dcc26705dc7cf144f5e0051)

Differential Revision: https://phabricator.services.mozilla.com/D55555

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/269ab6499d32ae39f0626858d05e13403787db69