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

* commit 'f4b20aeca4e818555c8c553ac1263a4f0f670d33':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I5665bb2fa41d10095af2ebe16a9b18a744f4f056