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

* commit '0b39f202aaedd3029e4270d2af7d58ba6634f9cd':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I28c3431879bb40b8426f7f2d8ce21197d4295fb6