Load extensions features file separately when needed

chrome/common/extensions/features/feature_provider.h

 // 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.
 
 #ifndef CHROME_COMMON_EXTENSIONS_FEATURES_FEATURE_PROVIDER_H_
 #define CHROME_COMMON_EXTENSIONS_FEATURES_FEATURE_PROVIDER_H_
 
 #include <string>
 
 namespace extensions {
 
 class Feature;
 
 // Implemented by classes that can vend features.
 class FeatureProvider {
  public:
   FeatureProvider() {}
   virtual ~FeatureProvider() {}
 
   // Returns the feature with the specified name.
   virtual Feature* GetFeature(const std::string& name) = 0;
+
+  // Returns all features described by this instance.
+  virtual std::set<std::string> GetAllFeatureNames() = 0;
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_COMMON_EXTENSIONS_FEATURES_FEATURE_PROVIDER_H_