Add comments support for PNG encoder

Bug: skia:6689
Change-Id: I6ccac8890f44724ccd6e20c8f4ea80bc99f9fa58
Reviewed-on: https://skia-review.googlesource.com/19080
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
3 files changed