[Chromoting] Enable jscompile for webapp unittests.

remoting/webapp/unittests/sinon_helpers.js

+// 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.
+
+var sinonHelpers = {};
+
+sinonHelpers.reset = function() {
+
+/**
+ * @param {Object} obj
+ * @param {string} method
+ * @return {sinon.TestStub}
+ * @suppress {reportUnknownTypes}
+ */
+sinon.$setupStub = function(obj, method) {
+  sinon.stub(obj, method);
+  obj[method].$testStub = /** @type {sinon.TestStub} */ (obj[method]);
+  return obj[method].$testStub;
+};
+
+};