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
am: 0f95eaef8e

* commit '0f95eaef8ec5a0e6e76d36095f405d9c646b0359':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I29c4033633839fc44b9de7ef56a965c5dcb3a7d6