Add DateExtensionEnabled field trial

BUG=301677
R=mad@chromium.org

Review URL: https://codereview.chromium.org/27003003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228283 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228283