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

* commit 'fd1852923233f8430da35126201245bcc06ac4e6':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I4d46b6b8fcc4bcf613b4d454e92c0cd4356784dc