Remove implementation of Panels on OSes other than ChromeOS.

chrome/test/data/extensions/api_test/window_open/panel_browsing_instance/panel.js

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-chrome.test.log('executing panel.js ...');
-
-// panel-subframe has to be navigated to another SiteInstance, so that in
-// --isolate-extensions mode it gets kicked out to another renderer process and
-// therefore has to go through the browser process when trying to find a frame
-// via window.open that gets called from panel-subframe.js (this will go through
-// BrowsingInstance checks in the browser process).
-chrome.test.getConfig(function(config) {
-  var baseUri = 'http://foo.com:' + config.testServer.port +
-      '/extensions/api_test/window_open/panel_browsing_instance/';
-
-  chrome.test.log('navigating panel-subframe ...');
-  var subframe = document.getElementById('panel-subframe-id');
-  subframe.src = baseUri +
-      'panel-subframe.html?extensionId=' + chrome.runtime.id;
-});