Remove third_party/polymer from .gitignore

third_party/polymer/v1_0/components/iron-overlay-behavior/iron-overlay-backdrop.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-overlay-manager.html">
-
-<!--
-`iron-overlay-backdrop` is a backdrop used by `Polymer.IronOverlayBehavior`. It should be a
-singleton.
-
-### Styling
-
-The following custom properties and mixins are available for styling.
-
-Custom property | Description | Default
--------------------------------------------|------------------------|---------
-`--iron-overlay-backdrop-background-color` | Backdrop background color                                     | #000
-`--iron-overlay-backdrop-opacity`          | Backdrop opacity                                              | 0.6
-`--iron-overlay-backdrop`                  | Mixin applied to `iron-overlay-backdrop`.                      | {}
-`--iron-overlay-backdrop-opened`           | Mixin applied to `iron-overlay-backdrop` when it is displayed | {}
--->
-
-<dom-module id="iron-overlay-backdrop">
-
-  <style>
-
-    :host {
-      position: fixed;
-      top: 0;
-      left: 0;
-      width: 100vw;
-      height: 100vh;
-      background-color: var(--iron-overlay-backdrop-background-color, #000);
-      opacity: 0;
-      transition: opacity 0.2s;
-
-      @apply(--iron-overlay-backdrop);
-    }
-
-    :host([opened]) {
-      opacity: var(--iron-overlay-backdrop-opacity, 0.6);
-
-      @apply(--iron-overlay-backdrop-opened);
-    }
-
-  </style>
-
-  <template>
-    <content></content>
-  </template>
-
-</dom-module>
-
-<script>
-
-(function() {
-
-  Polymer({
-
-    is: 'iron-overlay-backdrop',
-
-    properties: {
-
-      /**
-       * Returns true if the backdrop is opened.
-       */
-      opened: {
-        readOnly: true,
-        reflectToAttribute: true,
-        type: Boolean,
-        value: false
-      },
-
-      _manager: {
-        type: Object,
-        value: Polymer.IronOverlayManager
-      }
-
-    },
-
-    /**
-     * Appends the backdrop to document body and sets its `z-index` to be below the latest overlay.
-     */
-    prepare: function() {
-      if (!this.parentNode) {
-        Polymer.dom(document.body).appendChild(this);
-        this.style.zIndex = this._manager.currentOverlayZ() - 1;
-      }
-    },
-
-    /**
-     * Shows the backdrop if needed.
-     */
-    open: function() {
-      // only need to make the backdrop visible if this is called by the first overlay with a backdrop
-      if (this._manager.getBackdrops().length < 2) {
-        this._setOpened(true);
-      }
-    },
-
-    /**
-     * Hides the backdrop if needed.
-     */
-    close: function() {
-      // only need to make the backdrop invisible if this is called by the last overlay with a backdrop
-      if (this._manager.getBackdrops().length < 2) {
-        this._setOpened(false);
-      }
-    },
-
-    /**
-     * Removes the backdrop from document body if needed.
-     */
-    complete: function() {
-      // only remove the backdrop if there are no more overlays with backdrops
-      if (this._manager.getBackdrops().length === 0 && this.parentNode) {
-        Polymer.dom(this.parentNode).removeChild(this);
-      }
-    }
-
-  });
-
-})();
-
-</script>