[GlobalMutex] Do not assume the thread conflict happens in tests There isn't a guarantee that threads would always conflict with each other, so the assertion here would fail randomly. Overall: https://ci.chromium.org/ui/test/chromium/ninja%3A%2F%2Fthird_party%2Fangle%2Fsrc%2Ftests%3Aangle_unittests%2FGlobalMutexTest.ScopedOptionalGlobalMutexLockDisabled?q=+V%3Atest_suite%3Dangle_unittests+ Examples: https://chromium-swarm.appspot.com/task?id=623bb4e76b382911 https://chromium-swarm.appspot.com/task?id=62ba96ea430b8811 Fixed: angleproject:8194 Change-Id: Iaf2e5371a162d21f565e72f96d487d12c69f88c3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4606236 Auto-Submit: Zijie He <zijiehe@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>