Make SkFontMgr::onMakeFromStreamArgs pure virtual.

Ensure that it always exists and that implementers don't forget about it
and wind up with an incorrect default.

Change-Id: I383cb1eb8e15de49fbbb007d719837804aca33af
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299456
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
5 files changed