Add third_party NSS, SQLite, and zlib to Dart LICENSE file

BUG=
R=sgjesse@google.com

Review URL: https://codereview.chromium.org//657893004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41259 260f80e4-7a28-3924-810f-c04153c831b5