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

* commit '6fb6441a673f4b67d9dff7fcbbba7c4782fbd100':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I930fa163ade4760e90d143330b4e765462d73be7