add pagehide event listener on init for chrome target unload

[oyiptong]

Until we add functionality to WebChannels, In order for the chrome NewTabWebChannel bookkeeping to work, we need to send an event on pagehide.

Here's an example:

document.addEventListener("pagehide", function(e) {
  let msg = new window.CustomEvent("WebChannelMessageToChrome", {
    detail: {
      id: "dev-newtab",
      message: JSON.stringify({type: "pagehide"}),
    }
  });
  window.dispatchEvent(msg);
});