SkString: fix ::setUTF16

Change-Id: I7ab4af9ae55a43cc05f81b0c236f29a64beac982
Reviewed-on: https://skia-review.googlesource.com/111223
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
3 files changed