code: function DOMWalker(controller, callbackFilter, callbackNodeTest,
                     callbackResults) {
  
    this._controller = controller;
    this._callbackFilter = callbackFilter;
    this._callbackNodeTest = callbackNodeTest;
    this._callbackResults = callbackResults;
  }
docs: DOMWalker Constructor
  
  @param {MozMillController} controller
  MozMill controller of the window to operate on.
  @param {Function} callbackFilter
  callback-method to filter nodes
  @param {Function} callbackNodeTest
  callback-method to test accepted nodes
  @param {Function} callbackResults
  callback-method to process the results
  [optional - default: undefined]