Make more SkTypeface virtuals pure.

This ensures that proxy typefaces are really proxying these calls. Also,
sometimes the defaults aren't really the right thing.

Change-Id: Ifac417452136adca8152f4cd7670fd40553bd3b6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209105
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>