Remove third_party/polymer from .gitignore

third_party/polymer/v1_0/components/iron-menu-behavior/iron-menubar-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">
-<link rel="import" href="iron-menu-behavior.html">
-
-<script>
-
-  /**
-   * `Polymer.IronMenubarBehavior` implements accessible menubar behavior.
-   *
-   * @polymerBehavior Polymer.IronMenubarBehavior
-   */
-  Polymer.IronMenubarBehaviorImpl = {
-
-    hostAttributes: {
-      'role': 'menubar'
-    },
-
-    keyBindings: {
-      'left': '_onLeftKey',
-      'right': '_onRightKey'
-    },
-
-    _onUpKey: function(event) {
-      this._activateFocused(event.detail.keyboardEvent);
-    },
-
-    _onDownKey: function(event) {
-      this._activateFocused(event.detail.keyboardEvent);
-    },
-
-    _onLeftKey: function() {
-      this._focusPrevious();
-    },
-
-    _onRightKey: function() {
-      this._focusNext();
-    },
-
-    _onKeydown: function(event) {
-      if (this.keyboardEventMatchesKeys(event, 'up down left right esc enter')) {
-        return;
-      }
-
-      // all other keys focus the menu item starting with that character
-      this._focusWithKeyboardEvent(event);
-    }
-
-  };
-
-  /** @polymerBehavior Polymer.IronMenubarBehavior */
-  Polymer.IronMenubarBehavior = [
-    Polymer.IronMenuBehavior,
-    Polymer.IronMenubarBehaviorImpl
-  ];
-
-</script>