DO NOT MERGE Update libpng to 1.6.20 am: ed2e977a88
am: a2478bdf6e

* commit 'a2478bdf6eb37013e570b8dcfc7fcf8633350343':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I43ea27f0210343c19f37bbf9ecaca9b7389a9fcc