[Sync:USS] Add MAYBE prefix to explicit passphrase prefs test

Original CL missed MAYBE prefix and this could lead to test flakiness
on tsan tryjobs.

Bug: 922900
Change-Id: I1faf72c55b7942ad53b4f0e0bd9efdee979f427b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729633
Auto-Submit: Maksim Moskvitin <mmoskvitin@google.com>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683099}
1 file changed