GitLab

This adds the BUILD.gn file so GN can generate the ninja files for this
third_party project.

BUG=None
TEST=gn, ninja -C out/Default libpng
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/123613008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244161 0039d316-1c4b-4281-b951-d872f2087c98