we save the native fonts by their full path now. On macOS, there is no
such thing as a full filesystem path for a CGFont (or at least we don't track it),
so loading a capture falls back to the old logic of using the dummy font.

Differential Revision: https://phabricator.services.mozilla.com/D36604

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/dd501b160508c7dedf613941135707435b2e42c1