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This allows unit tests that uses FakeConciergeClient to test the
case where base::nullopt response is passed to these callbacks
without needing to modify the existing tests at all.

BUG=b:136128691
TEST=try

Change-Id: I25b31be7f060b419184e891413d21b5a645ac46b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894042Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711635}