Add relTypes() to WebPrerender.h.

We don't want to enable rel=next prerenders in blink until downstream
chrome support is ready (so they don't default on); this patch allows
downstream chrome patches to land before we start parsing them.

R=japhet@chromium.org
BUG=329963

Review URL: https://codereview.chromium.org/144863004

git-svn-id: svn://svn.chromium.org/blink/trunk@166761 bbb929c8-8fbe-4397-9dbb-9b2b20218538