Add a histogram to track Chrome's UpdatePolicy in the field.

BUG=346494
TEST=Run dev chrome and confirm (after 45s) that chrome://histograms shows:
GoogleUpdate.UpdatePolicyIsOverridden => 0 (false)
GoogleUpdate.EffectivePolicy => 1 (AUTOMATIC_UPDATES)

R=asvitkine@chromium.org, grt@chromium.org, isherman@chromium.org

Review URL: https://codereview.chromium.org/178473004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254595 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254595