Add Media Router drop-down-button.

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

Index: chrome/browser/resources/media_router/elements/drop_down_button/drop_down_button.css
diff --git a/chrome/browser/resources/media_router/elements/drop_down_button/drop_down_button.css b/chrome/browser/resources/media_router/elements/drop_down_button/drop_down_button.css
new file mode 100644
index 0000000000000000000000000000000000000000..f8a0f56d563c6c9ace55cbae01b9244cd55702e9
--- /dev/null
+++ b/chrome/browser/resources/media_router/elements/drop_down_button/drop_down_button.css
@@ -0,0 +1,26 @@
+/* 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. */
+
+#drop-down {
+  background: white;
+  background-image: -webkit-image-set(
+    url(../icon/drop-down-arrow.png) 1x,
+    url(../icon/drop-down-arrow2x.png) 2x);
+  background-repeat: no-repeat;
+  border: 1px solid rgb(192, 195, 198);
+  height: 18px;
+  width: 18px;
+}
+
+#drop-down:hover {
+  background-image: -webkit-image-set(
+    url(../icon/drop-down-arrow-hover.png) 1x,
+    url(../icon/drop-down-arrow-hover2x.png) 2x);
+}
+
+#drop-down.showing {
+  background-image: -webkit-image-set(
+    url(../icon/drop-down-arrow-showing.png) 1x,
+    url(../icon/drop-down-arrow-showing2x.png) 2x);
+}