Minor improvements of task list

appengine/swarming/elements/res/imp/botlist/bot-list.html

 <!--
   Copyright 2016 The LUCI Authors. All rights reserved.
   Use of this source code is governed under the Apache License, Version 2.0
   that can be found in the LICENSE file.
 
   This in an HTML Import-able file that contains the definition
   of the following elements:
 
     <bot-list>
 
@@ skipping 205 matching lines @@
   (function(){
     var UNKNOWN = "unknown";
     // see dynamic-table for more information on specialColumns, headerMap,
     // columnMap, and specialSort
     var specialColumns = ["id", "task"];
 
     var headerMap = {
       // "id" and "task" are special, so they don't go here. They have their
       // headers hard-coded above.
       "android_devices": "Android Devices",
-      "cores": "Cores",
-      "cpu": "CPU",
-      "device": "Non-android Device",
-      "device_os": "Device OS",
-      "device_type": "Device Type",
-      "disk_space": "Free Space (MB)",
-      "gpu": "GPU",
-      "os": "OS",
-      "pool": "Pool",
-      "status": "Status",
-      "xcode_version": "XCode Version",
       "battery_health": "Battery Health",
       "battery_level": "Battery Level (%)",
       "battery_status": "Battery Status",
       "battery_temperature": "Battery Temp (°C)",
       "battery_voltage": "Battery Voltage (mV)",
       "bot_temperature": "Bot Temp (°C)",
+      "cores": "Cores",
+      "cpu": "CPU",
+      "device": "Non-android Device",
+      "device_os": "Device OS",
       "device_temperature": "Device Temp (°C)",
+      "device_type": "Device Type",
+      "disk_space": "Free Space (MB)",
+      "first_seen": "First Seen",
+      "gpu": "GPU",
+      "last_seen": "Last Seen",
+      "os": "OS",
+      "pool": "Pool",
+      "status": "Status",
+      "xcode_version": "XCode Version",
     };
 
     var columnMap = {
       android_devices: function(bot) {
         var devs = this._attribute(bot, "android_devices", "0");
         if (this._verbose) {
           return devs.join(" | ") + " devices available";
         }
         // max() works on strings as long as they can be coerced to Number.
         return Math.max(...devs) + " devices available";
@@ skipping 255 matching lines @@
         if (!device.okay) {
           return "bad-device";
         }
         return "";
       },
 
     });
   })();
   </script>
 </dom-module>