[net] Remove logic for fixing up Gzip encoding type for downloads.

net/base/filename_util_unsafe.h

-// 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.
-
-#ifndef NET_BASE_FILENAME_UTIL_UNSAFE_H_
-#define NET_BASE_FILENAME_UTIL_UNSAFE_H_
-
-#include <string>
-
-#include "base/files/file_path.h"
-#include "base/strings/string16.h"
-#include "net/base/net_export.h"
-
-class GURL;
-
-namespace net {
-
-// Extract extension from FilePath generated by GenerateFileName(), but without
-// replacing illegal characters. Does not depend on ICU.
-NET_EXPORT base::FilePath::StringType GenerateFileExtensionUnsafe(
-    const GURL& url,
-    const std::string& content_disposition,
-    const std::string& referrer_charset,
-    const std::string& suggested_name,
-    const std::string& mime_type,
-    const std::string& default_name);
-
-}  // namespace net
-
-#endif  // NET_BASE_FILENAME_UTIL_UNSAFE_H_