[Extensions] Add extension feature generation code

extensions/common/features/api_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/api_feature.h"
 
 namespace extensions {
 
 APIFeature::APIFeature() {}
 
 APIFeature::~APIFeature() {
 }
 
 bool APIFeature::Validate(std::string* error) {
   if (!SimpleFeature::Validate(error))
     return false;
 
-  if (contexts()->empty()) {
+  if (contexts().empty()) {
     *error = name() + ": API features must specify at least one context.";
     return false;
   }
 
   return true;
 }
 
 }  // namespace extensions