make skmatrix getmapproc private

Make SkMatrix MapXYProc MapPtsProc and friends private.
Code search turned up no clients in chromium, google3,
android. Fingers crossed.

R:reed@google.com
Bug: skia:6898
Change-Id: Iee20fe5150499215a09f67cc6f117b685f38f455
Reviewed-on: https://skia-review.googlesource.com/62140
Commit-Queue: Cary Clark <caryclark@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
12 files changed