[SiteChip] Add UMA, finish wiring site chip experiment up to the omnibox.

BUG=315944
R=msw@chromium.org

Review URL: https://codereview.chromium.org/110893004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240652 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240652