Remove third_party/polymer from .gitignore

third_party/polymer/v1_0/components/neon-animation/animations/slide-down-animation.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="../neon-animation-behavior.html">
-<link rel="import" href="../web-animations.html">
-
-<!--
-`<slide-down-animation>` animates the transform of an element from `translateY(-100%)` to `none`.
-The `transformOrigin` defaults to `50% 0`.
-
-Configuration:
-```
-{
-  name: 'slide-down-animation',
-  node: <node>,
-  transformOrigin: <transform-origin>,
-  timing: <animation-timing>
-}
-```
--->
-
-<script>
-
-  Polymer({
-
-    is: 'slide-down-animation',
-
-    behaviors: [
-      Polymer.NeonAnimationBehavior
-    ],
-
-    configure: function(config) {
-      var node = config.node;
-
-      if (config.transformOrigin) {
-        this.setPrefixedProperty(node, 'transformOrigin', config.transformOrigin);
-      } else {
-        this.setPrefixedProperty(node, 'transformOrigin', '50% 0');
-      }
-
-      this._effect = new KeyframeEffect(node, [
-        {'transform': 'translateY(-100%)'},
-        {'transform': 'none'}
-      ], this.timingFromConfig(config));
-
-      return this._effect;
-    }
-
-  });
-
-</script>