title: Cannot get download ID in chrome extension
body: <p>I am working with a chrome extension, in that the download code was like this</p>
  
  <pre><code>chrome.downloads.download({
    url: "http://meenkada.net67.net/test.html",
    filename: "test.html" // Optional
  });
  </code></pre>
  
  <p>But with this I did not get the download ID. So, I modified the code as below</p>
  
  <pre><code>function DownloadCallback()
  {
  alert('hi');
  
  }
  
  chrome.downloads.download({
   url: "http://meenkada.net67.net/test.html",
    filename: "test.html" // Optional
  },function DownloadCallback);
  </code></pre>
  
  <p>but I am not able to get the download ID. Its for opening the downloaded file. Please help</p>