Fix DesktopMediaPickerViews to handle background parent_web_contents

chrome/common/extensions/docs/examples/api/desktopCapture/app.js

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 function gotStream(stream) {
   console.log("Received local stream");
   var video = document.querySelector("video");
   video.src = URL.createObjectURL(stream);
   localstream = stream;
   stream.onended = function() { console.log("Ended"); };
@@ skipping 20 matching lines @@
 document.querySelector('#start').addEventListener('click', function(e) {
   pending_request_id = chrome.desktopCapture.chooseDesktopMedia(
       ["screen", "window"], onAccessApproved);
 });
 
 document.querySelector('#cancel').addEventListener('click', function(e) {
   if (pending_request_id != null) {
     chrome.desktopCapture.cancelChooseDesktopMedia(pending_request_id);
   }
 });
+
+document.querySelector('#startFromBackgroundPage')
+    .addEventListener('click', function(e) {
+      chrome.runtime.sendMessage(
+          {}, function(response) { console.log(response.farewell); });
+    });