Add File::Duplicate to duplicate a file handle.

base/files/file.h

Index: base/files/file.h
diff --git a/base/files/file.h b/base/files/file.h
index 7b6366c1c2edaa2c6ccb695f2f075ff83f0d002b..51f90a96236fa01e6aa9656db7e847a8b0c90e0c 100644
--- a/base/files/file.h
+++ b/base/files/file.h
@@ -287,6 +287,13 @@ class BASE_EXPORT File {
   // 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 unlink the
+  // underlying file on close. 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)