[Extensions] Move Feature channel code into //extensions

extensions/common/features/complex_feature.h

 // 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.
 
 #ifndef EXTENSIONS_COMMON_FEATURES_COMPLEX_FEATURE_H_
 #define EXTENSIONS_COMMON_FEATURES_COMPLEX_FEATURE_H_
 
 #include <memory>
 #include <set>
 #include <string>
@@ skipping 25 matching lines @@
 
   Availability IsAvailableToContext(const Extension* extension,
                                     Context context,
                                     const GURL& url,
                                     Platform platform) const override;
 
   bool IsIdInBlacklist(const std::string& extension_id) const override;
   bool IsIdInWhitelist(const std::string& extension_id) const override;
 
  protected:
-  // extensions::Feature:
-  std::string GetAvailabilityMessage(AvailabilityResult result,
-                                     Manifest::Type type,
-                                     const GURL& url,
-                                     Context context) const override;
-
+  // Feature:
   bool IsInternal() const override;
 
  private:
   FeatureList features_;
 
   DISALLOW_COPY_AND_ASSIGN(ComplexFeature);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_FEATURES_COMPLEX_FEATURE_H_