DO NOT MERGE Update libpng to 1.6.20 am: ed2e977a88 am: a2478bdf6e am: 7509e3018b am: 7fc8b69ecc  -s ours am: f4b20aeca4
am: fe9b944aeb

* commit 'fe9b944aeb93cb03b40875b43bdf4188782b4c31':

Change-Id: I9bdbbe4f946df0c221e1c8bf53376ca25965c21a