MD Settings: Eliminate use of ES6 for Chrome OS

ui/webui/resources/cr_elements/network/cr_network_list.js

Index: ui/webui/resources/cr_elements/network/cr_network_list.js
diff --git a/ui/webui/resources/cr_elements/network/cr_network_list.js b/ui/webui/resources/cr_elements/network/cr_network_list.js
index e5de1abd3501f99ab432c7151d2f92b8cd823817..26cddbe00f8243d8b23fb4cc7900ac0129c72ebf 100644
--- a/ui/webui/resources/cr_elements/network/cr_network_list.js
+++ b/ui/webui/resources/cr_elements/network/cr_network_list.js
@@ -67,7 +67,7 @@ Polymer({
    * @private
    */
   getItems_: function() {
-    let customItems = this.customItems.slice();
+    var customItems = this.customItems.slice();
     // Flag the first custom item with isFirstCustomItem = true.
     if (customItems.length > 0)
       customItems[0].isFirstCustomItem = true;