Add <link rel=next> support to prerender.

IEs prerender implementation supports rel=next link elements. Let's
also parse them, and based on a field trial launch these as
prerenders.

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

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=250280

Review URL: https://codereview.chromium.org/113803003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250558 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250558