Add support for rel=next to Prerender in Blink.

<link rel=next ...> elements are also used to drive prerenders. Let's
parse them and pass them up to our embedder.

BUG=329963
R=japhet@chromium.org,abarth,tburkard,davidben,jochen

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=164241

Review URL: https://codereview.chromium.org/116183005

git-svn-id: svn://svn.chromium.org/blink/trunk@167099 bbb929c8-8fbe-4397-9dbb-9b2b20218538