Remove implementation of Panels on OSes other than ChromeOS.

chrome/test/data/extensions/api_test/window_open/panel_browsing_instance/panel-subframe.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.
-
-// If panel-subframe is (expectedly) in the same BrowsingInstance as
-// background-subframe, then window.open should find the existing iframe.
-//
-// Otherwise window.open will cause a new tab to be opened and we will
-// find this out and fail the test when comparing newNumberOfTabs and
-// oldNumberOfTabs in background.js.
-//
-// Note that we can't verify if |w| is a new window by looking
-// at |w.parent|, because this leads to an error similar to:
-//   Uncaught SecurityError: Blocked a frame with origin "http://foo.com:36559"
-//   from accessing a frame with origin
-//   "chrome-extension://jmanjdlcdjhgbdccaamcmmcilljchoad".  The frame
-//   requesting access has a protocol of "http", the frame being accessed has a
-//   protocol of "chrome-extension". Protocols must match.
-var w = window.open('http://blah', 'background-subframe-name')
-
-// Quick and very dirty query parameters parsing... ok only for test code.
-extensionId = window.location.href.substr(
-    window.location.href.lastIndexOf('=') + 1);
-
-// Let background.js know that it can now calculate newNumberOfTabs.
-chrome.runtime.sendMessage(extensionId, { windowOpened: true });
-