cbuildbot: plumb snapshot revision flag

Plumb the --cbb_snapshot_revision flag that will be passed
to post-submit builders inside of the --cbb_extra_args
flag. This will be parsed and set by the cros/swarming
recipe when triggered by Findit.

BUG=chromium:923572
TEST=unittests and tryjob

Change-Id: Ic31bbcc7e953d839cc4c2a43305c359fb21e99c7
Reviewed-on: https://chromium-review.googlesource.com/1477770
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Gregory Meinke <gmeinke@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 06e5041..aeb2129 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -486,6 +486,10 @@
                           default=[],
                           help='Specify an explicit list of packages to build '
                                'for integration with Findit.')
+  group.add_remote_option('--cbb_snapshot_revision', type='string',
+                          dest='cbb_snapshot_revision', default=None,
+                          help='Snapshot manifest revision to sync to '
+                               'for building.')
 
   parser.add_argument_group(group)