DO NOT MERGE Update libpng to 1.6.20 am: 0b39f202aa  -s ours am: 9795c8684c am: d0107515e0 am: 7d9fa2ce0f  -s ours am: fd386f8f05 am: 79bd206ce6 am: 557bd959eb am: 9dfb357e98  -s ours am: 1c6b279894
am: cc5a2eaea1

* commit 'cc5a2eaea1d0334e0cfded0a1d56956881fb880c':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: If03c24d9c0559fac602e1a104e9bc733a94a3262