Adds truncate to FileStream.

net/base/file_stream_posix.cc

 // 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.
 
 // For 64-bit file access (off_t = off64_t, lseek64, etc).
 #define _FILE_OFFSET_BITS 64
 
 #include "net/base/file_stream.h"
 
 #include <sys/types.h>
@@ skipping 183 matching lines @@
         continue;
       return MapErrorCode(errno);
     }
     total_bytes_written += res;
     buf += res;
     len -= res;
   }
   return total_bytes_written;
 }
 
+int64 FileStream::Truncate(int64 bytes) {
+  if (!IsOpen())
+    return ERR_UNEXPECTED;
+
+  // We better be open for reading.
+  DCHECK(open_flags_ & base::PLATFORM_FILE_WRITE);
+
+  // Seek to the position to truncate from.
+  int64 seek_position = Seek(FROM_BEGIN, bytes);
+  if (seek_position != bytes)
+    return ERR_UNEXPECTED;
+
+  // And truncate the file.
+  int result = ftruncate(file_, bytes);
+  return result == 0 ? seek_position : MapErrorCode(errno);
+}
+
 }  // namespace net