Add a fake cast extension for testing.

chrome/test/data/extensions/tray_cast/background.js

+var backgroundSetup = {}
+
+tryCreateActivity = function(id, title, tabId) {
+  if (id === undefined || title === undefined || tabId === undefined)
+    return null;
+
+  return {
+    "id": id,
+    "title": title,
+    "tabId": tabId
+  };
+}
+
+var receiversActivities = [];
+// Add a new receiver. |activityTitle| and |activityTabId| are optional
+// parameters.
+addReceiver = function(id, receiverName, activityTitle, activityTabId) {
+  receiversActivities.push({
+    "activity": tryCreateActivity(id, activityTitle, activityTabId),
+    "receiver": {
+      "id": id,
+      "name": receiverName
+    }
+  });
+}
+// Required API method.
+getMirrorCapableReceiversAndActivities = function() {
+  if (this !== backgroundSetup)
+    throw 'this !== backgroundSetup';
+
+  return receiversActivities;
+}
+
+var stopMirroringReason = "";
+var stopMirroringCalled = false;
+wasStopMirroringCalledWithUserStop = function() {
+  return stopMirroringCalled && stopMirroringReason == 'user-stop';
+}
+// Required API method.
+stopMirroring = function(reason) {
+  if (this !== backgroundSetup)
+    throw 'this !== backgroundSetup';
+
+  stopMirroringReason = reason;
+  stopMirroringCalled = true;
+  for (item of receiversActivities) {
+    if (item.activity != null)
+      item.activity = null;
+  }
+}
+
+var launchTabId = 1;
+var launchTabTitle = "Fake Cast";
+var launchDesktopMirroringReceiverId = "";
+getLaunchDesktopMirroringReceiverId = function() {
+  return launchDesktopMirroringReceiverId;
+}
+// Required API method.
+launchDesktopMirroring = function(receiverId) {
+  if (this !== backgroundSetup)
+    throw 'this !== backgroundSetup';
+
+  launchDesktopMirroringReceiverId = receiverId;
+
+  for (item of receiversActivities) {
+    if (item.receiver.id == receiverId) {
+      item.activity =
+        tryCreateActivity(receiverId, launchTabId, launchTabTitle);
+      break;
+    }
+  }
+}