DO NOT MERGE Update libpng to 1.6.20 am: 0b39f202aa  -s ours
am: 9795c8684c

* commit '9795c8684ca5f889ab66265cca1e001cd400398c':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I537720b5bcf2c441c51e59a52619dc8435269702