DO NOT MERGE Update libpng to 1.6.20 am: ed2e977a88 am: a2478bdf6e am: 7509e3018b am: 7fc8b69ecc  -s ours am: f4b20aeca4 am: 6fb6441a67
am: 27cc8e987d  -s ours

* commit '27cc8e987d5c6ea61646354650c37c4e4302f77a':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I46a9ae2b20ac5ad29c44f2d23cb68e87975ff9eb