Create prerender event histograms by origin & experiment.

R=asvitkine@chromium.org, davidben@chromium.org

Review URL: https://codereview.chromium.org/100133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238440 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238440