This adds the pieces needed to accomplish this, and
although clients could do it, I figured it would be
nice to expose as a universal tool (on TextBlob).
Bug: skia:
Change-Id: Id5d61744973de2da75049d33d40e1dc442c2442c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201601
Reviewed-by: Ben Wagner <bungeman@google.com>