Implement InstanceID API functions.

chrome/test/data/extensions/api_test/instance_id/incognito/test.js

-// Copyright 2014 The Chromium Authors. All rights reserved.
+// Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 chrome.test.runTests([
   function testIncognito() {
-    chrome.gcm.register(["Sender"], function(registrationId) {
+    chrome.instanceID.getID(function(id) {
       chrome.test.assertEq(chrome.runtime.lastError != undefined,
                            chrome.extension.inIncognitoContext);
       chrome.test.succeed();
     });
   }
 ]);