Remove third_party/polymer from .gitignore

third_party/polymer/v1_0/components/iron-form-element-behavior/iron-form-element-behavior.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
--->
-
-<link rel="import" href="../polymer/polymer.html">
-
-<!--
-Enables a custom element to be included in an `iron-form`.
--->
-<script>
-
-  /**
-
-  @demo demo/index.html
-  @polymerBehavior
-
-  */
-  Polymer.IronFormElementBehavior = {
-
-    properties: {
-      /**
-       * Fired when the element is added to an `iron-form`.
-       *
-       * @event iron-form-element-register
-       */
-
-      /**
-       * Fired when the element is removed from an `iron-form`.
-       *
-       * @event iron-form-element-unregister
-       */
-
-      /**
-       * The name of this element.
-       */
-      name: {
-        type: String
-      },
-
-      /**
-       * The value for this element.
-       */
-      value: {
-        notify: true,
-        type: String
-      },
-
-      /**
-       * The form that the element is registered to.
-       */
-      _parentForm: {
-        type: Object
-      }
-    },
-
-    attached: function() {
-      // Note: the iron-form that this element belongs to will set this
-      // element's _parentForm property when handling this event.
-      this.fire('iron-form-element-register');
-    },
-
-    detached: function() {
-      if (this._parentForm) {
-        this._parentForm.fire('iron-form-element-unregister', {target: this});
-      }
-    }
-
-  };
-
-</script>