[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 20 matching lines @@
   if (extension && !extension->manifest()->value()->HasKey(name()))
     return CreateAvailability(NOT_PRESENT, extension->GetType());
 
   return CreateAvailability(IS_AVAILABLE);
 }
 
 bool ManifestFeature::Validate(std::string* error) {
   if (!SimpleFeature::Validate(error))
     return false;
 
-  if (extension_types()->empty()) {
+  if (extension_types().empty()) {
     *error = name() + ": Manifest features must specify at least one " +
              "value for extension_types.";
     return false;
   }
 
-  if (!contexts()->empty()) {
+  if (!contexts().empty()) {
     *error = name() + ": Manifest features do not support contexts.";
     return false;
   }
 
   return true;
 }
 
 }  // namespace extensions