Move ASCIIToWide and ASCIIToUTF16 to utf_string_conversions.h. I've found it...

net/url_request/url_request_context.h

 // Copyright (c) 2010 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.
 
 // This class represents contextual information (cookies, cache, etc.)
 // that's useful when processing resource requests.
 // The class is reference-counted so that it can be cleaned up after any
 // requests that are using it have been completed.
 
 #ifndef NET_URL_REQUEST_URL_REQUEST_CONTEXT_H_
 #define NET_URL_REQUEST_URL_REQUEST_CONTEXT_H_
 #pragma once
 
 #include "base/non_thread_safe.h"
 #include "base/ref_counted.h"
-#include "base/string_util.h"
 #include "net/base/cookie_store.h"
 #include "net/base/host_resolver.h"
 #include "net/base/net_log.h"
 #include "net/base/ssl_config_service.h"
 #include "net/base/transport_security_state.h"
 #include "net/ftp/ftp_auth_cache.h"
 #include "net/proxy/proxy_service.h"
 
 namespace net {
 class CookiePolicy;
@@ skipping 70 matching lines @@
 
   // Gets the value of 'Accept-Charset' header field.
   const std::string& accept_charset() const { return accept_charset_; }
 
   // Gets the value of 'Accept-Language' header field.
   const std::string& accept_language() const { return accept_language_; }
 
   // Gets the UA string to use for the given URL.  Pass an invalid URL (such as
   // GURL()) to get the default UA string.  Subclasses should override this
   // method to provide a UA string.
-  virtual const std::string& GetUserAgent(const GURL& url) const {
-    return EmptyString();
-  }
+  virtual const std::string& GetUserAgent(const GURL& url) const;
 
   // In general, referrer_charset is not known when URLRequestContext is
   // constructed. So, we need a setter.
   const std::string& referrer_charset() const { return referrer_charset_; }
   void set_referrer_charset(const std::string& charset) {
     referrer_charset_ = charset;
   }
 
  protected:
   friend class base::RefCountedThreadSafe<URLRequestContext>;
@@ skipping 19 matching lines @@
   // The charset of the referrer where this request comes from. It's not
   // used in communication with a server but is used to construct a suggested
   // filename for file download.
   std::string referrer_charset_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(URLRequestContext);
 };
 
 #endif  // NET_URL_REQUEST_URL_REQUEST_CONTEXT_H_