Remove third_party/polymer from .gitignore

third_party/polymer/v1_0/components/iron-test-helpers/iron-test-helpers.html

-<!--
-@license
-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
--->
-<script src="mock-interactions.js"></script>
-<script src="test-helpers.js"></script>
-<script>
-  (function(global) {
-    'use strict';
-
-    var TestHelpers = global.TestHelpers = global.TestHelpers || {};
-
-    TestHelpers.flushAsynchronousOperations = flushAsynchronousOperations;
-    TestHelpers.forceXIfStamp = forceXIfStamp;
-    TestHelpers.fireEvent = fireEvent;
-  })(this);
-</script>