Make SkTypefacePlayback use smart pointers.

This clarifies the ownership of the typefaces as well as removes a use
of SkRefCnt_SafeAssign which is currently a code smell.

Change-Id: I8fec541f71f555c2182b77870979ece87b501901
Reviewed-on: https://skia-review.googlesource.com/140249
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
5 files changed