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

* commit '89d19aa8f1552bb9d49f541628af1b9aae6a80bb':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I5281367f06015395500b5330ba36f295a0f603fa