DO NOT MERGE Update libpng to 1.6.20 am: 0b39f202aa  -s ours am: 9795c8684c am: d0107515e0 am: 7d9fa2ce0f  -s ours am: fd386f8f05 am: 79bd206ce6 am: 557bd959eb am: 9dfb357e98  -s ours am: 1c6b279894
am: 51e10a60b1

* commit '51e10a60b1e847d492e7c50769d479cb1ea1edd1':

Change-Id: Icb07e79dfa1d48b9d3d707dcf4cf65ebc4d4f085