Add histograms for prerender rel types.

Since we have an associated finch experiment, these two histograms
should be good enough for us to catch information on the population
of rel types of prerenders as added and run.

R=tburkard@chromium.org,davidben@chromium.org,isherman@chromium.org
BUG=329963

Review URL: https://codereview.chromium.org/98603015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250784 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250784