Make OAuth client id accessible to new ui w/o hardcoding

appengine/swarming/elements/res/imp/common/swarming-app.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:
 
     <swarming-app>
 
@@ skipping 64 matching lines @@
     </style>
     <app-header-layout>
       <app-header fixed>
         <app-toolbar>
           <div class="title left">[[name]]</div>
           <paper-spinner-lite class="left" active="[[busy]]"></paper-spinner-lite>
 
           <a class="left" href="/newui/">Home</a>
           <a class="left" href="/newui/botlist">Bot List</a>
           <div class="flex"></div>
-          <!-- TODO(kjlubick) Remove this hard-coded client-id.-->
           <auth-signin
               class="right"
-              client-id="20770472288-t5smpbpjptka4nd888fv0ctd23ftba2o.apps.googleusercontent.com"
+              client-id="[[client_id]]"
               auth-headers="{{auth_headers}}"
               signed-in="{{signed_in}}">
           </auth-signin>
         </app-toolbar>
       </app-header>
       <div class="main-content">
         <content></content>
       </div>
     </app-header-layout>
 
   </template>
   <script>
     Polymer({
       is: 'swarming-app',
       properties: {
+        client_id: {
+          type: String,
+        },
         auth_headers: {
           type: Object,
           notify: true,
         },
         signed_in: {
           type: Boolean,
           value: false,
           notify:true,
         },
 
@@ skipping 13 matching lines @@
   <style>
       * {
         font-family: sans-serif;
       }
       /* Only style anchor tags that are actually linking somewhere.*/
       a[href] {
         color: #1F78B4;
       }
   </style>
 </dom-module>