Introduce chrome.shell.createWindow stub API for app_shell

chrome/common/extensions/api/_api_features.json

 // Copyright (c) 2012 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.
 
 // See c/c/e/features/* to understand this file, in particular feature.h,
 // simple_feature.h, and base_feature_provider.h.
 //
 // Note that specifying "web_page", "blessed_web_page", or "all" as a context
 // type will require manually updating chrome/renderer/resources/dispatcher.cc.
 
@@ skipping 625 matching lines @@
     "contexts": ["blessed_extension"]
   },
   "serial": {
     "dependencies": ["permission:serial"],
     "contexts": ["blessed_extension"]
   },
   "sessions": {
     "dependencies": ["permission:sessions"],
     "contexts": ["blessed_extension"]
   },
+  // TODO(jamescook): Move this to app_shell _api_features.json once that file
+  // exists.
+  "shell": {
+    "contexts": ["blessed_extension"],
+    "extension_types": ["platform_app"]
+  },
   "signedInDevices": {
     "dependencies": ["permission:signedInDevices"],
     "contexts": ["blessed_extension"]
   },
   "socket": {
     "dependencies": ["permission:socket"],
     "contexts": ["blessed_extension"]
   },
   "sockets.tcp": {
     "dependencies": ["manifest:sockets"],
@@ skipping 157 matching lines @@
     "internal": true,
     "channel": "stable",
     "dependencies": ["permission:webview"],
     "contexts": ["blessed_extension"]
   },
   "windows": {
     "dependencies": ["api:tabs"],
     "contexts": ["blessed_extension"]
   }
 }