move set-text-matrix into private

Had no external callers, and was poorly named.

Bug: skia:
Change-Id: I5bed6853e9aa3b31fa106a436a73297bf55f0503
Reviewed-on: https://skia-review.googlesource.com/16155
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed