Deflake DateTrayTest ShowTasksComboModel.

Bug: b:277268122
Change-Id: If9f3215f0b4758ca5f9ca01f5d56f4c95d8d6b97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4558348


Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Yulun Wu <yulunwu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148694}