Clean up SkString reference counting a bit.

BUG=skia:7107

Change-Id: I47072bf31b902c79dbb850179ff6d35940de3e63
Reviewed-on: https://skia-review.googlesource.com/54720
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
2 files changed