Remove third_party/polymer from .gitignore

third_party/polymer/v1_0/components/more-routing/driver/mock.html

-<!--
-Copyright (c) 2015 The Polymer Project Authors. All rights reserved.
-This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
-The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
-The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
-Code distributed by Google as part of the polymer project is also
-subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
--->
-<link rel="import" href="../driver.html">
-
-<script>
-(function(scope) {
-var MoreRouting = scope.MoreRouting = scope.MoreRouting || {};
-MoreRouting.MockDriver = MockDriver;
-
-/** A mock driver for use in your tests. */
-function MockDriver() {
-  MoreRouting.Driver.apply(this, arguments);
-}
-MockDriver.prototype = Object.create(MoreRouting.Driver.prototype);
-
-MockDriver.prototype.navigateToUrl = function navigateToUrl(url) {
-  this.setCurrentPath(url);
-};
-
-})(window);
-</script>