[Extensions] Move Feature channel code into //extensions

extensions/common/features/feature_channel.h

 // Copyright 2013 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.
 
-#ifndef CHROME_COMMON_EXTENSIONS_FEATURES_FEATURE_CHANNEL_H_
-#define CHROME_COMMON_EXTENSIONS_FEATURES_FEATURE_CHANNEL_H_
+#ifndef EXTENSIONS_COMMON_FEATURES_FEATURE_CHANNEL_H_
+#define EXTENSIONS_COMMON_FEATURES_FEATURE_CHANNEL_H_
 
 #include "base/macros.h"
 
 namespace version_info {
 enum class Channel;
 }
 
 namespace extensions {
 
 // Gets the current channel as seen by the Feature system.
@@ skipping 14 matching lines @@
   ~ScopedCurrentChannel();
 
  private:
   version_info::Channel original_channel_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedCurrentChannel);
 };
 
 }  // namespace extensions
 
-#endif  // CHROME_COMMON_EXTENSIONS_FEATURES_FEATURE_CHANNEL_H_
+#endif  // EXTENSIONS_COMMON_FEATURES_FEATURE_CHANNEL_H_