Make headless_shell target compile for Windows

headless/lib/headless_content_main_delegate.h

 // Copyright 2015 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 HEADLESS_LIB_HEADLESS_CONTENT_MAIN_DELEGATE_H_
 #define HEADLESS_LIB_HEADLESS_CONTENT_MAIN_DELEGATE_H_
 
 #include <memory>
+#include <string>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "content/public/app/content_main_delegate.h"
 #include "headless/lib/browser/headless_platform_event_source.h"
 #include "headless/lib/headless_content_client.h"
 
 namespace base {
 class CommandLine;
 }
@@ skipping 33 matching lines @@
   HeadlessPlatformEventSource platform_event_source_;
 
   std::unique_ptr<HeadlessBrowserImpl> browser_;
 
   DISALLOW_COPY_AND_ASSIGN(HeadlessContentMainDelegate);
 };
 
 }  // namespace headless
 
 #endif  // HEADLESS_LIB_HEADLESS_CONTENT_MAIN_DELEGATE_H_