I'm seeing sporadic failures from a few days ago, but now suddenly 4 builds in a row have failed.

I'm seeing this stack trace, but I'm not sure if it's the cause of the failure of a side-effect.

[547:22275:1120/120149.199912:FATAL:thread_restrictions.cc(78)] Check failed: !g_blocking_disallowed.Get().Get(). To allow //base sync primitives in a scope where blocking is disallowed use ScopedAllowBaseSyncPrimitivesOutsideBlockingScope.
0   Chromium Framework                  0x00000001149e37bf base::debug::StackTrace::StackTrace(unsigned long) + 31
1   Chromium Framework                  0x00000001146dbe48 logging::LogMessage::~LogMessage() + 696
2   Chromium Framework                  0x0000000114933e14 base::ScopedAllowBaseSyncPrimitives::ScopedAllowBaseSyncPrimitives() + 340
3   Chromium Framework                  0x00000001147dd854 base::StackSamplingProfiler::~StackSamplingProfiler() + 244
4   Chromium Framework                  0x0000000114acf4b6 ThreadProfiler::StartPeriodicSamplingCollection() + 934
5   Chromium Framework                  0x0000000114ad1225 base::internal::Invoker<base::internal::BindState<void (ThreadProfiler::*)(), base::WeakPtr<ThreadProfiler> >, void ()>::RunOnce(base::internal::BindStateBase*) + 549
6   Chromium Framework                  0x000000011469549d base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask*) + 797
7   Chromium Framework                  0x000000011489c364 base::sequence_manager::internal::ThreadControllerImpl::DoWork(base::sequence_manager::internal::ThreadControllerImpl::WorkType) + 2340
8   Chromium Framework                  0x00000001148a13e5 base::internal::Invoker<base::internal::BindState<void (base::sequence_manager::internal::ThreadControllerImpl::*)(base::sequence_manager::internal::ThreadControllerImpl::WorkType), base::WeakPtr<base::sequence_manager::internal::ThreadControllerImpl>, base::sequence_manager::internal::ThreadControllerImpl::WorkType>, void ()>::Run(base::internal::BindStateBase*) + 581
9   Chromium Framework                  0x000000011469549d base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask*) + 797
10  Chromium Framework                  0x000000011472a871 base::MessageLoopImpl::RunTask(base::PendingTask*) + 817
11  Chromium Framework                  0x000000011472bf20 base::MessageLoopImpl::DoWork() + 1296
12  Chromium Framework                  0x000000011472f8ff base::MessagePumpDefault::Run(base::MessagePump::Delegate*) + 719
13  Chromium Framework                  0x000000011472986a base::MessageLoopImpl::Run(bool) + 426
14  Chromium Framework                  0x00000001147e189d base::RunLoop::Run() + 685
15  Chromium Framework                  0x000000011492c2b3 base::Thread::Run(base::RunLoop*) + 451
16  Chromium Framework                  0x000000011492ce6e base::Thread::ThreadMain() + 1614
17  Chromium Framework                  0x0000000114a0cb8e base::(anonymous namespace)::ThreadFunc(void*) + 206
18  libsystem_pthread.dylib             0x00007fffe968e93b _pthread_body + 180
19  libsystem_pthread.dylib             0x00007fffe968e887 _pthread_body + 0
20  libsystem_pthread.dylib             0x00007fffe968e08d thread_start + 13

The failures stopped for no apparent reason. Still it might be worth investigating what was going on.

Likely related to previous  Issue 774682  (crashes with the sampling profiler) but since not reproducible anymore closing as WontFix. Thanks for reporting.

The responsible change was reverted on Nov 21.