cbuildbot.py: add metadata_dump command line option

In order to land upcoming metadata_dump changes, it is necessary that
cbuildbot.py be able to accept a metadata_dump argument without
complaint, even if it does not use this argument.

BUG=chromium:356930
TEST=None

Change-Id: Ib54beccf2b8efffe724ad756f0ee065ba9cb49df
Reviewed-on: https://chromium-review.googlesource.com/205221
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 35fb4de..da46afd 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -1301,6 +1301,8 @@
   group.add_remote_option('--validation_pool', default=None,
                           help=('Path to a pickled validation pool. Intended '
                                 'for use only with the commit queue.'))
+  group.add_remote_option('--metadata_dump', default=None,
+                          help=('This option is not yet used.'))
   group.add_remote_option('--mock-tree-status', dest='mock_tree_status',
                           default=None, action='store',
                           help=('Override the tree status value that would be '