Add cross domain tracking to Tracker object

So that pages that link to known partners can allow those partners to
track full user journeys, this commit adds the necessary UA incantations
to link domains and separate accounts together.

This is for Universal Analytics only.

javascripts/govuk/analytics/google-analytics-universal-tracker.js

@@ -89,6 +89,29 @@
     });
   };
 
+  /*
+   https://developers.google.com/analytics/devguides/collection/analyticsjs/cross-domain
+   trackerId - the UA account code to track the domain against
+   name      - name for the tracker
+   domain    - the domain to track
+  */
+  GoogleAnalyticsUniversalTracker.prototype.addLinkedTrackerDomain = function(trackerId, name, domain) {
+    sendToGa('create',
+             trackerId,
+             'auto',
+             {'name': name});
+    // Load the plugin.
+    sendToGa('require', 'linker');
+    sendToGa(name + '.require', 'linker');
+
+    // Define which domains to autoLink.
+    sendToGa('linker:autoLink', [domain]);
+    sendToGa(name + '.linker:autoLink', [domain]);
+
+    sendToGa(name + '.set', 'anonymizeIp', true);
+    sendToGa(name + '.send', 'pageview');
+  };
+
   // https://developers.google.com/analytics/devguides/collection/analyticsjs/custom-dims-mets
   GoogleAnalyticsUniversalTracker.prototype.setDimension = function(index, value) {
     sendToGa('set', 'dimension' + index, String(value));

javascripts/govuk/analytics/tracker.js

@@ -46,5 +46,12 @@
     this.classic.setCustomVariable(index, value, name, scope);
   };
 
+  /*
+   Add a beacon to track a page in another GA account on another domain.
+   */
+  Tracker.prototype.addLinkedTrackerDomain = function(trackerId, name, domain) {
+    this.universal.addLinkedTrackerDomain(trackerId, name, domain);
+  };
+
   GOVUK.Tracker = Tracker;
 })();

spec/unit/analytics/TrackerSpec.js

@@ -71,4 +71,25 @@ describe("GOVUK.Tracker", function() {
     });
   });
 
+  describe('when adding a linked domain', function() {
+    beforeEach(function() {
+      tracker = new GOVUK.Tracker(this.config);
+    });
+
+    it('adds a linked domain to universal only', function() {
+      window._gaq = [];
+      tracker.addLinkedTrackerDomain('1234', 'test', 'www.example.com');
+
+      expect(window._gaq).toEqual([]);
+      var allArgs = window.ga.calls.allArgs()
+      expect(allArgs).toContain(['create', '1234', 'auto', {'name': 'test'}]);
+      expect(allArgs).toContain(['require', 'linker']);
+      expect(allArgs).toContain(['test.require', 'linker']);
+      expect(allArgs).toContain(['linker:autoLink', ['www.example.com']]);
+      expect(allArgs).toContain(['test.linker:autoLink', ['www.example.com']]);
+      expect(allArgs).toContain(['test.set', 'anonymizeIp', true]);
+      expect(allArgs).toContain(['test.send', 'pageview']);
+    });
+  });
+
 });