Add Media Router drop-down-button.

chrome/browser/resources/media_router/elements/drop_down_button/drop_down_button.js

Index: chrome/browser/resources/media_router/elements/drop_down_button/drop_down_button.js
diff --git a/chrome/browser/resources/media_router/elements/drop_down_button/drop_down_button.js b/chrome/browser/resources/media_router/elements/drop_down_button/drop_down_button.js
new file mode 100644
index 0000000000000000000000000000000000000000..e56caead82c1c7f4c125c810567a97caa14a8027
--- /dev/null
+++ b/chrome/browser/resources/media_router/elements/drop_down_button/drop_down_button.js
@@ -0,0 +1,19 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+Polymer('drop-down-button', {
+  created: function() {
+    /** @type {boolean} */
+    this.iconShown = false;
+  },
+
+  /**
+   * Fires a drop-down-button-click event. This is called when the element is
+   * clicked. Switches the value of |iconShown|.
+   */
+  onButtonClick: function() {
+    this.iconShown = !this.iconShown;
+    this.fire('drop-down-button-click');
+  }
+});