DO NOT MERGE Update libpng to 1.6.20 am: cf8a579007 am: a0b5142c6c am: 34dcc2af83  -s ours am: 3aec76d873  -s ours am: 9cac6eee10 am: c34faaeec7 am: 8ecc2c0512  -s ours am: 89d19aa8f1 am: 9a9e74b2ec am: 0f95eaef8e am: fd18529232  -s ours am: aa0664c5f8
am: f724ab3959

* commit 'f724ab39595265a257d8a1c161fcc839b4af9066':

Change-Id: I8c948719cf8a7dbde9e5bb1254a51d260761d63b