Add File::Duplicate to duplicate a file handle.

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 "build/build_config.h"
 #if defined(OS_WIN)
 #include <windows.h>
@@ skipping 270 matching lines @@
   // POSIX-specific semantics:
   //  * Locks are advisory only.
   //  * Within a process, locking the same file (by the same or new handle)
   //    will succeed.
   //  * Closing any descriptor on a given file releases the lock.
   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();
+
   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.
   static std::string ErrorToString(Error error);
@@ skipping 55 matching lines @@
 #endif
 
   Error error_details_;
   bool created_;
   bool async_;
 };
 
 }  // namespace base
 
 #endif  // BASE_FILES_FILE_H_