Add histograms for the Contextual Search field trial.

Related change: https://chrome-internal-review.googlesource.com/#/c/154947/

BUG=341765

Review URL: https://codereview.chromium.org/164753005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252261 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252261