Revert rebaselining for libpng

It seems like not all of the tests need to be rebaselined. Some failures appear
to be geniune problems with the updated libpng. Further investigation is
required.

BUG=303648

Review URL: https://codereview.chromium.org/35503003

git-svn-id: svn://svn.chromium.org/blink/trunk@160343 bbb929c8-8fbe-4397-9dbb-9b2b20218538