Bug 1878199 - patch 2 - If the FontFaceImpl has a user-font entry, hold its...

Bug 1878199 - patch 2 - If the FontFaceImpl has a user-font entry, hold its lock during Add/Remove font-set operations.  a=dmeehan

This should fix the intermittently-reported race here, by ensuring that access into
the FontFaceImpl from GetUserFontSets(), called by the main thread, cannot race with
changes being made by the AddFontSet()/RemoveFontSet() methods.

(If the FontFaceImpl doesn't have an mUserFontEntry yet, then these methods don't
need to lock, as only the owning thread will be touching it.)

Original Revision: https://phabricator.services.mozilla.com/D207296

Differential Revision: https://phabricator.services.mozilla.com/D208098