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

appengine/swarming/elements/res/imp/index/swarming-index.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-index>
 
     Swarming Index is the landing page for the Swarming UI.
     It will have links to all other pages and a high-level overview of the fleet.
 
   Usage:
 
     <swarming-index></swarming-index>
 
+  This is a top-level element.
+
   Properties:
-    None.  This is a top-level element.
+    client_id: String, will be set by server-side template evaluation.
 
   Methods:
     None.
 
   Events:
     None.
 -->
 
 <link rel="import" href="/res/imp/bower_components/iron-ajax/iron-ajax.html">
 
 <link rel="import" href="/res/imp/common/swarming-app.html">
 
 <dom-module id="swarming-index">
   <template>
     <style include="swarming-app-style">
 
     </style>
 
     <swarming-app
+      client_id="[[client_id]]"
       auth_headers="{{auth_headers}}"
       name="Swarming"
       busy="[[busy]]">
 
       <iron-ajax id="request"
         url="/_ah/api/swarming/v1/server/details"
         headers="[[auth_headers]]"
         handle-as="json"
         last-response="{{serverDetails}}"
         loading="{{busy}}">
       </iron-ajax>
 
       <h1>HELLO WORLD</h1>
 
       <div>Server Version: [[serverDetails.server_version]]</div>
 
     </swarming-app>
 
   </template>
   <script>
     Polymer({
       is: 'swarming-index',
 
       properties: {
+        client_id: {
+          type: String,
+        },
+
         auth_headers: {
           type: Object,
           observer: "signIn",
         },
 
         serverDetails: {
           type: String,
         }
       },
 
       signIn: function(){
         this.$.request.generateRequest();
       },
 
     });
   </script>
 </dom-module>