Make File::Duplicate() const

base/files/file.h

 // 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.
 
 #ifndef BASE_FILES_FILE_H_
 #define BASE_FILES_FILE_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 272 matching lines @@
   Error Lock();
 
   // Unlock a file previously locked.
   Error Unlock();
 
   // Returns a new object referencing this file for use within the current
   // process. Handling of FLAG_DELETE_ON_CLOSE varies by OS. On POSIX, the File
   // object that was created or initialized with this flag will have unlinked
   // the underlying file when it was created or opened. On Windows, the
   // underlying file is deleted when the last handle to it is closed.
-  File Duplicate();
+  File Duplicate() const;
 
   bool async() const { return async_; }
 
 #if defined(OS_WIN)
   static Error OSErrorToFileError(DWORD last_error);
 #elif defined(OS_POSIX)
   static Error OSErrorToFileError(int saved_errno);
 #endif
 
   // Converts an error value to a human-readable form. Used for logging.
@@ skipping 29 matching lines @@
   bool created_;
   bool async_;
 
   DISALLOW_COPY_AND_ASSIGN(File);
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_FILE_H_