Move SkPngEncoder into public API

Bug: 713862
Change-Id: I45068ed39affe41ffe0f29bf42c5ea1d9b0247ba
Reviewed-on: https://skia-review.googlesource.com/15897
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
3 files changed