youTubeIFrameInject.js

//  Create the port needed to communicate with the parent extension.
this.initializePort = function() {
  this.port = chrome.runtime.connect({
    name: 'youTubeIFrameConnectRequest'
  });

  //  The extension can request the *exact* time of YouTube's video player.
  //  Respond with that value, but also include a timestamp to account for the time it takes to send the postMessage.
  this.port.onMessage.addListener(function(message) {
    if (message === 'getCurrentTimeHighPrecision') {
      var currentTime = this.videoStream === null ? 0 : this.videoStream.currentTime;

      this.port.postMessage({
        timestamp: performance.now(),
        currentTimeHighPrecision: currentTime
      });
    }
  }.bind(this));
}.bind(this);