move SkMatrix anonymous affine enum to private

enum members aren't used by SkMatrix.h or by
clients outside of Skia.

R: reed@google.com, bungeman@google.com
Bug: skia:6898
Change-Id: I6873b4106e5ffe354caf5ec18cc613910304fa13
Reviewed-on: https://skia-review.googlesource.com/59160
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Cary Clark <caryclark@skia.org>
5 files changed