Clean-up coding style

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"
 
+#include <string>
+
 namespace extensions {
 
 APIFeature::APIFeature()
     : internal_(false), blocked_in_service_worker_(false) {}
 
 APIFeature::~APIFeature() {
 }
 
 bool APIFeature::IsInternal() const {
   return internal_;
@@ skipping 11 matching lines @@
   value->GetBoolean("internal", &internal_);
   value->GetBoolean("blocked_in_service_worker", &blocked_in_service_worker_);
 
   if (GetContexts()->empty())
     return name() + ": API features must specify at least one context.";
 
   return std::string();
 }
 
 }  // namespace extensions