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: I32fdd1860e9ea5f8354d60ade8790df3bef7de1c