DO NOT MERGE Update libpng to 1.6.20 am: cf8a579007 am: a0b5142c6c am: 34dcc2af83  -s ours am: 3aec76d873  -s ours am: 9cac6eee10 am: c34faaeec7 am: 8ecc2c0512  -s ours am: 89d19aa8f1
am: 9a9e74b2ec

* commit '9a9e74b2ec3fa1f1e9ee126869fa9ff677d6d7d1':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: Ib29a06e1f3f955ce2a3f4416ec753e8e89f16e60