DO NOT MERGE Update libpng to 1.6.20 am: 0b39f202aa  -s ours am: 9795c8684c
am: d0107515e0

* commit 'd0107515e075179036f0f220874bf7eec0cbb3b5':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I94c330086568d910457745485ea6539b96992164