DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/tests/pngunknown-discard b/tests/pngunknown-discard
index f1f02d4..c96dab3 100755
--- a/tests/pngunknown-discard
+++ b/tests/pngunknown-discard
@@ -1,2 +1,2 @@
 #!/bin/sh
-exec ./pngunknown default=discard "${srcdir}/pngtest.png"
+exec ./pngunknown --strict default=discard "${srcdir}/pngtest.png"