_initializeHistoryListener() {

		this._historyListener = ({state}) => {
			this._navigateWithHistoryState({
				state,
				path  : this._history.getPath(),
				type  : History.events.POPSTATE, // Forward/back.
				check : true, // Only navigate if frame is ours.
			});
		};

		this._history = new History();
		var init = () => this._history.on(this._historyListener);

		// Need to go _after_ 'load' callbacks complete.
		// Safari fires a 'popstate' on load (RED-67600).
		// https://developer.mozilla.org/en-US/docs/Web/Events/popstate
		if (document.readyState === 'complete'){
			init();
		} else {
			window.addEventListener('load', ()=>setTimeout(init,0));
		}
	}
Initializes us to listen to back button events. When the user presses the back button, the history
listener will be called and cause a navigate() event.