SkPngCodec clean-ups

Use png_read_row() instead of png_read_rows(1).
All png_read_rows() does is call png_read_row() in
a loop.  This comes from Leon's comment in:
https://codereview.chromium.org/1671003004/

Also there is a bit of refactoring.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1675843002

Review URL: https://codereview.chromium.org/1675843002
1 file changed