cros_generate_update_payload: fix metadata_size_file

In a previous CL:912102, the metadata size file was not passed to the correct
command. This CL fixes it.

BUG=chromium:808495
TEST='cros_generate_update_payload --out_metadata_size_file ./file' write into
./file the size of the metadata during payload generation.

Change-Id: I0941deebbb6457daa39ca2adb742482fd06f3715
Reviewed-on: https://chromium-review.googlesource.com/919823
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
1 file changed