message: scope: fix {add,remove}Document method
diff: @@ -2,6 +2,8 @@ const utils = require('./utils');
  const events = require('./utils/events');
  const signals = require('./utils/Signals').new();
  +const { getWindow } = require('./utils/window');
  +
  const scope = {
  signals,
  events,
  @@ -16,7 +18,7 @@ const scope = {
  // do nothing if document is already known
  if (utils.contains(scope.documents, doc)) { return false; }
  - win = win || scope.getWindow(doc);
  + win = win || getWindow(doc);
  scope.documents.push(doc);
  events.documents.push(doc);
  @@ -33,7 +35,7 @@ const scope = {
  removeDocument: function (doc, win) {
  const index = scope.documents.indexOf(doc);
  - win = win || scope.getWindow(doc);
  + win = win || getWindow(doc);
  events.remove(win, 'unload', scope.onWindowUnload);
  