[base] Fix flakiness in MessagePumpLibeventTest.QuitWatcher

The TSAN log shows that the issue is that the test's WriteFDWrapper
is using the |pipefds_| on |io_thread_| unsynchronized with the main
thread closing the pipe from TearDown().

Joining the test's |io_thread_| solves the issue.

R=fdoray@chromium.org

Bug: 955440
Change-Id: Ifc4966518d2e6abce4d0a3cd8b301e3241a057f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1990340
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729406}
1 file changed