Adds truncate to FileStream.

net/base/file_stream_posix.cc

Index: net/base/file_stream_posix.cc
===================================================================
--- net/base/file_stream_posix.cc	(revision 11127)
+++ net/base/file_stream_posix.cc	(working copy)
@@ -201,4 +201,21 @@
   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