Make SkYUVAIndex publicly accessible

Change-Id: I3da7c566cdf9256e57ac7e7f30fee18b9c1a144d
Reviewed-on: https://skia-review.googlesource.com/c/159460
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
8 files changed