DO NOT MERGE Update libpng to 1.6.20 am: cf8a579007
am: a0b5142c6c

* commit 'a0b5142c6cb915175194aa85d16b8bd3de6d0e94':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I0c766e2a9938ba7846a6a95f674fce3bbf4aec10