Adds truncate to FileStream.

net/base/file_stream.h

 // Copyright (c) 2008 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.
 
 // This file defines FileStream, a basic interface for reading and writing files
 // synchronously or asynchronously with support for seeking to an offset.
 // Note that even when used asynchronously, only one operation is supported at
 // a time.
 
 #ifndef NET_BASE_FILE_STREAM_H_
@@ skipping 91 matching lines @@
   // In the case of an asychronous write, the memory pointed to by |buf| must
   // remain valid until the callback is notified.  However, it is valid to
   // destroy or close the file stream while there is an asynchronous write in
   // progress.  That will cancel the write and allow the buffer to be freed.
   //
   // This method should not be called if the stream was opened READ_ONLY.
   //
   // You can pass NULL as the callback for synchronous I/O.
   int Write(const char* buf, int buf_len, CompletionCallback* callback);
 
+  // Truncates the file to be |bytes| length. This is only valid for writable
+  // files. After truncation the file stream is positioned at |bytes|. The new
+  // position is retured, or a value < 0 on error.
+  int64 Truncate(int64 bytes);
+
  private:
   class AsyncContext;
   friend class AsyncContext;
 
   // This member is used to support asynchronous reads.  It is non-null when
   // the FileStream was opened with PLATFORM_FILE_ASYNC.
   scoped_ptr<AsyncContext> async_context_;
 
   base::PlatformFile file_;
   int open_flags_;
 
   DISALLOW_COPY_AND_ASSIGN(FileStream);
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_FILE_STREAM_H_