This was added in a previous patch but it seems that we don't need it if invalid glyphs can be treated as blank glyph cache items.

Depends on D68857

Differential Revision: https://phabricator.services.mozilla.com/D68858

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/e3f5c752e57fb82c03a9e8fb288907599b6d9489