[Extensions] Add extension feature generation code

extensions/common/features/manifest_feature.cc

 // 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.
 
 #include "extensions/common/features/manifest_feature.h"
 
 #include "extensions/common/manifest.h"
 
 namespace extensions {
 
@@ skipping 21 matching lines @@
     return CreateAvailability(NOT_PRESENT, extension->GetType());
 
   return CreateAvailability(IS_AVAILABLE);
 }
 
 std::string ManifestFeature::Parse(const base::DictionaryValue* value) {
   std::string error = SimpleFeature::Parse(value);
   if (!error.empty())
     return error;
 
-  if (extension_types()->empty()) {
+  if (extension_types().empty()) {
     return name() + ": Manifest features must specify at least one " +
         "value for extension_types.";
   }
 
   if (value->HasKey("contexts"))
     return name() + ": Manifest features do not support contexts.";
 
   return std::string();
 }
 
 }  // namespace extensions