title: chrome.tabCapture.capture returning NULL stream
body: <p>I have been searching around a bit, but didn't find any solution to this issue.
  I'm not sure why but the <code>stream</code> which I'm getting is <code>null</code>.</p>
  
  <p>As you can see in the code bellow, I'm not using <code>browserAction</code> but instead I'm sending a message to the extension and then calling <code>requestCurrentTabCapture</code></p>
  
  <pre><code>chrome.runtime.onConnect.addListener(function(port) {
    port.onMessage.addListener(function(msg) {
  
      switch(msg.type){
        case 'SS_TAB_REQUEST':
          alert("SS_TAB_REQUEST");
          requestCurrentTabCapture(port, msg);
          break;
        case 'SS_TAB_CANCEL':
          break;
        default:
      }
    });
  });
  
  function requestCurrentTabCapture(port, msg){
    //chrome.tabs.getCurrent(function(tab) {
      //chrome.tabs.getSelected(null, function(tab) {
      chrome.tabs.query({active: true}, function(tab) {
          alert(tab);
          chrome.tabCapture.capture(
            {
              video: true, audio: true,
              videoConstraints: {
               mandatory: {
                 chromeMediaSource: 'tab',
                 minWidth: 16,
                 minHeight: 9,
                 maxWidth: 1280,
                 maxHeight: 720,
                 maxFrameRate: 60,  // Note: Frame rate is variable (0 &lt;= x &lt;= 60).
               },
             },
            },
            function(stream) {
              alert(stream); //&lt;-- NULL
              if (!stream) {
               console.error('Error starting tab capture: ' + chrome.runtime.lastError.message || 'UNKNOWN'));
             }
            }
          );      
      });
  
  
      chrome.tabCapture.onStatusChanged.addListener(function (info){
              alert("status: "+info.status); //&lt;-- not shown
              alert("tabId: "+info.tabId); //&lt;-- not shown
              alert("fullscreen: "+info.fullscreen); //&lt;-- not shown
      });
  }
  </code></pre>
  
  <p>Permissions are as follows:</p>
  
  <pre><code>"permissions": [
   "desktopCapture",
   "&lt;all_urls&gt;",
   "tabs",
  "activeTab",
   "tabCapture"
  ]
  </code></pre>