DO NOT MERGE Update libpng to 1.6.20 am: ed2e977a88 am: a2478bdf6e am: 7509e3018b
am: 7fc8b69ecc  -s ours

* commit '7fc8b69ecc55078132fa72e413b45b0d649e2286':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I4ff0b4a0c5c2f383e01854b03f51f1276e04bc75