background.js

/* background page, responsible for actually choosing media */
chrome.runtime.onMessageExternal.addListener(function (message, sender, callback) {

  switch(message.type) {
    case 'getScreen': {
      var pending = chrome.desktopCapture.chooseDesktopMedia(
        message.options || ['screen', 'window'],
        sender.tab,
        function (streamid) {
          // communicate this string to the app so it can call getUserMedia with it
          message.type = 'gotScreen';
          message.sourceId = streamid;
          callback(message);
          return false;
        }
      );
      return true; // retain callback for chooseDesktopMedia result
    }

    case 'cancelGetScreen': {
      chrome.desktopCapture.cancelChooseDesktopMedia(message.request);
      message.type = 'canceledGetScreen';
      callback(message);
      return false; //dispose callback
    }

    case 'isInstalled': {
      callback({ installed: true, extensionId: chrome.runtime.id });
      return false; //dispose callback
    }
  }
});