Make 'License' field in third-party metadata required

This will simplify the addition of a tool to check licenses for the purpose of
the Android WebView build.

See also http://codereview.chromium.org/10821103.

BUG=138921

Review URL: https://chromiumcodereview.appspot.com/10827099

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/hunspell_dictionaries@149334 4ff67af0-8c30-449e-8e8b-ad334ec8d88c