DO NOT MERGE Update libpng to 1.6.20 am: 0b39f202aa  -s ours am: 9795c8684c am: d0107515e0
am: 7d9fa2ce0f  -s ours

* commit '7d9fa2ce0fdd41ad7545b18e5f428733d9e5c2f1':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I8dd2a9746c3570c5f9913ed0d29a84a48c22d27b