Make File::Duplicate() const

base/files/file_posix.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 "base/files/file.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <stdint.h>
 #include <sys/stat.h>
@@ skipping 352 matching lines @@
 File::Error File::Lock() {
   SCOPED_FILE_TRACE("Lock");
   return CallFcntlFlock(file_.get(), true);
 }
 
 File::Error File::Unlock() {
   SCOPED_FILE_TRACE("Unlock");
   return CallFcntlFlock(file_.get(), false);
 }
 
-File File::Duplicate() {
+File File::Duplicate() const {
   if (!IsValid())
     return File();
 
   SCOPED_FILE_TRACE("Duplicate");
 
   PlatformFile other_fd = dup(GetPlatformFile());
   if (other_fd == -1)
     return File(OSErrorToFileError(errno));
 
   File other(other_fd);
@@ skipping 140 matching lines @@
   return !HANDLE_EINTR(fsync(file_.get()));
 #endif
 }
 
 void File::SetPlatformFile(PlatformFile file) {
   DCHECK(!file_.is_valid());
   file_.reset(file);
 }
 
 }  // namespace base