Update the version of libpng used by Skia tests

Update the version of libpng used by Skia tests

The version of libpng we were using has some security vulnerabilities.
Update to a newer version - 1.6.20 (http://sourceforge.net/p/libpng/code/ci/047737496a77eeb97f9991919dac08ca3c149711/).

Update pnglibconf.h. The new version is a copy of the newer version of
the file, with our /* custom settings */ tacked onto the end. (The
custom settings are unchanged from the last version.)

Update the LICENSE. The new version is a copy of the LICENSE that
libpng now includes as a separate file.

Update the README file to reflect the change in source of the LICENSE
file.

BUG=skia:4573
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1453373003

Review URL: https://codereview.chromium.org/1453373003