Delete unnecessary PrerenderRelType enum

We can just use WebPrerenderRelType directly and we won't need to worry about
them getting out of sync.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/399473002

git-svn-id: svn://svn.chromium.org/blink/trunk@178218 bbb929c8-8fbe-4397-9dbb-9b2b20218538