Add SkCharacterization creation helper to GrContextThreadSafeProxy

Change-Id: I8ad7cf335f2b586cf501eaa70573690fbbd53efa
Reviewed-on: https://skia-review.googlesource.com/106105
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
18 files changed