Clarify ownership of typefaces in implementation.

This converts a number of ownership semantics by using sk_sp and
unique_ptr instead of comments.

Change-Id: I4c584a5a8ea54fd22a4d23deddb5ce844f883117
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/197206
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Herb Derby <herb@google.com>
11 files changed