Make Tot the "real" default value of roll_deps.py's ref option

Thanks to Ic386d62addec124e9c4c01c3a0bb4469e9193de9, we can roll to
working tree of chromium code base.
But without "default=ReferenceMode.Tot", the default value is None,
and thus the script does not update the origin/main.
So this CL sets Tot as the default value.

Bug: N/A
Change-Id: I6355ccc54bb929297d46251eda6ecf1c5fa7fd0f
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/4933565
Commit-Queue: Lingqi Chi <lingqi@chromium.org>
Reviewed-by: Ken Okada <kenoss@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
diff --git a/scripts/deps/roll_deps.py b/scripts/deps/roll_deps.py
index fb163a3..d624457 100755
--- a/scripts/deps/roll_deps.py
+++ b/scripts/deps/roll_deps.py
@@ -64,6 +64,7 @@
         '--ref',
         type=ReferenceMode,
         choices=list(ReferenceMode),
+        default=ReferenceMode.Tot,
         help='Defaults to tot. '
         'If tot, fetch origin/main of Chromium repository and use it. '
         'If working-tree, use working tree as is.')