DO NOT MERGE Update libpng to 1.6.20 am: ed2e977a88 am: a2478bdf6e
am: 7509e3018b

* commit '7509e3018b5b53b6435b3682556c07ac3916d976':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I3bb56c1134b72610554c451fafa2f38170cd24d5