Move apps/shell to extensions/shell.

extensions/shell/common/api/_api_features.json

 // Copyright 2014 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.
 
-// This features file defines extension APIs implemented under src/apps/shell.
+// This features file defines extension APIs implemented under src/extensions/shell.
 // See extensions/common/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.
 
 {
   "shell": {
     "channel": "dev",
     "contexts": ["blessed_extension"],
     "extension_types": ["platform_app"]
   }
 }