DO NOT MERGE Update libpng to 1.6.20 am: cf8a579007 am: a0b5142c6c
am: 34dcc2af83  -s ours

* commit '34dcc2af8385e79dadfb7f829d7388ff17f1e398':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I5bcdb53e8f04a2ad35a5395064d697961100ea17