add helpers to SkFont (and SkTypeface)

Bug: skia:2664
Change-Id: I215c951ae2d46fa7ee0ccbff490375fc60694100
Reviewed-on: https://skia-review.googlesource.com/c/165780
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
4 files changed