Fix DesktopMediaPickerViews to handle background parent_web_contents

chrome/common/extensions/docs/examples/api/desktopCapture/background.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.
 
 chrome.app.runtime.onLaunched.addListener(function() {
   chrome.app.window.create('index.html', {
     bounds: {
       width: 700,
       height: 600
     }
   });
 });
+
+chrome.runtime.onMessage.addListener(function(message, sender, sendResponse) {
+  chrome.desktopCapture.chooseDesktopMedia(
+      ["screen", "window"],
+      function(id) {
+        sendResponse({"id": id});
+      });
+});