Add protobuf library to third_party.

This is going to be used by SyzyASAN for enhanced crash reporting data.

BUG=
R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/130390045

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2270 15e8cca8-e42c-11de-a347-f34a4f72eb7d