GitLab

Aligning with the POSIX impl in preparation for
https://chromium-review.googlesource.com/c/chromium/src/+/1283010

Also adds two trivial ScopedAllowBaseSyncPrimitivesForTesting around
existing thread joins in tests (more advanced changes were extracted
to precursor CLs).

Bug: 707362
Change-Id: If4c4c49b4a20554518ca4a25ae1ea16b84d3a510
Reviewed-on: https://chromium-review.googlesource.com/c/1324370
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609013}