src/: Update the remaining include paths of string_split.h to its new location.

net/http/http_request_headers.cc

 // Copyright (c) 2012 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 "net/http/http_request_headers.h"
 
 #include "base/logging.h"
+#include "base/string_util.h"
 #include "base/stringprintf.h"
-#include "base/string_split.h"
-#include "base/string_util.h"
+#include "base/strings/string_split.h"
 #include "base/values.h"
 #include "net/http/http_util.h"
 
 namespace net {
 
 const char HttpRequestHeaders::kGetMethod[] = "GET";
 const char HttpRequestHeaders::kAcceptCharset[] = "Accept-Charset";
 const char HttpRequestHeaders::kAcceptEncoding[] = "Accept-Encoding";
 const char HttpRequestHeaders::kAcceptLanguage[] = "Accept-Language";
 const char HttpRequestHeaders::kAuthorization[] = "Authorization";
@@ skipping 228 matching lines @@
        it != headers_.end(); ++it) {
     if (key.length() == it->key.length() &&
         !base::strncasecmp(key.data(), it->key.data(), key.length()))
       return it;
   }
 
   return headers_.end();
 }
 
 }  // namespace net