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

* commit 'c34faaeec7ce5e1a5a7a2de12f0dec56c75c33da':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I86aed7034232e616d36a2089a22092f2f7a05572