cbuildbot.py: add metadata_dump command line option

Cherry-pick of https://chromium-review.googlesource.com/#/c/205221/

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.


Change-Id: Ic4eaa943072f3de95a3ccda702e9c9ff8e0ad316
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 770eddd..402b3dc 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -1274,6 +1274,8 @@
   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 '