vaadin-grid not visible within a paper-header-panel
<p>I am trying to display a table data using vaadin-grid, embedded within a paper-header-panel element. The data is supposed to come from a Rest call. The Rest client used is  and I am binding the last-response attribute with the 'items' attribute of the vaadin-grid. When I am inspecting the HTML using firebug, I am able to see the table is properly getting created, but in the frontend nothing is getting visible.</p>

<p>Below is my custom element: 'aws-api-gateway-call'
</p>

<pre><code>&lt;template&gt;
    &lt;iron-ajax
        auto
        url="{{url}}"
        handle-as="json"
        last-response="{{handleResponse}}"&gt;
     &lt;/iron-ajax&gt;

    &lt;vaadin-grid items="{{handleResponse}}" selection-mode="multi"&gt;
        &lt;table&gt;
          &lt;colgroup&gt;
              &lt;col name="fileName"&gt;
              &lt;col name="titleName"&gt;
              &lt;col name="artist"&gt;
              &lt;col name="albumName"&gt;
              &lt;col name="action"&gt;
          &lt;/colgroup&gt;
          &lt;thead&gt;
            &lt;tr&gt;
              &lt;th style="z-index: 10"&gt;MP3 File Name&lt;/th&gt;
              &lt;th&gt;Title Name&lt;/th&gt;
              &lt;th&gt;Artist Name&lt;/th&gt;
              &lt;th&gt;Album Name&lt;/th&gt;
              &lt;th&gt;Action&lt;/th&gt;
            &lt;/tr&gt;
            &lt;/thead&gt;
        &lt;/table&gt;
      &lt;/vaadin-grid&gt;

&lt;/template&gt;

&lt;script&gt;
    Polymer({
          is: "aws-api-gateway-call",

          properties : {
              hostname:{
                  value: 'test',
                  notify: true
              },
              stage:{
                  value: 'test',
                  notify: true
              },
              method:{
                  value: 'test',
                  notify: true
              },
              url:{
                  computed: 'computeUrl(hostname, stage, method)'
              }
          },              
          computeUrl: function(hostname, stage, method){
              console.log('URL to call--&gt;' + [hostname, stage, method].join('/'))
              return [hostname, stage, method].join('/')
          }
        });
    &lt;/script&gt;
</code></pre>

<p></p>

<p>And, here is my :</p>

<pre><code>&lt;!DOCTYPE html&gt;
&lt;html&gt;
&lt;head&gt;
&lt;meta charset="ISO-8859-1"&gt;
&lt;title&gt;An Web-app to customize MP3 songs' metadata&lt;/title&gt;
&lt;link rel="shortcut icon" href="img/vector-black-ipod-10170961.jpg" /&gt;

    &lt;script type="text/javascript" src="bower_components/webcomponentsjs/webcomponents.js"&gt;&lt;/script&gt;
    &lt;script type="text/javascript" src="bower_components/prefixfree/prefixfree.js"&gt;&lt;/script&gt;
    &lt;link rel="import" href="bower_components/paper-header-panel/paper-header-panel.html"&gt;
    &lt;link rel="import" href="bower_components/iron-flex-layout/iron-flex-layout-classes.html"&gt;
    &lt;link rel="import" href="elements/api_calls.html"&gt;
    &lt;link rel="stylesheet" href="css/main.css"&gt;
&lt;/head&gt;
&lt;body class="fullbleed layout vertical"&gt;

        &lt;paper-header-panel class="flex"&gt;
            &lt;div class="paper-header ipodHeaderPanel"&gt;My Ipod Customizer Application&lt;/div&gt;
            &lt;div class="content"&gt;

                &lt;aws-api-gateway-call 
                    hostname="https://&lt;acct-id&gt;.execute-api.us-west-2.amazonaws.com" 
                    stage="mp3file" 
                    method="MP3MetadataReadMicroService"&gt;                           
                &lt;/aws-api-gateway-call&gt;

            &lt;/div&gt;
        &lt;/paper-header-panel&gt;


&lt;/body&gt;
&lt;/html&gt;
</code></pre>

<p><em>Can you please help me understand the issue? And how to fix it?</em></p>