Seems to have been flaky for some time now. See https://build.chromium.org/p/chromium.webrtc.fyi/builders/Mac%20Tester/builds/41285/steps/content_browsertests/logs/stdio

[9270:775:0717/074313.731098:1867352939822:INFO:CONSOLE(105)] "Receiving answer...", source: http://127.0.0.1:52903/media/webrtc_test_common.js (105)
[9273:37635:0717/074313.732191:1867354031960:WARNING:video_send_stream.cc(1134)] Transmitting payload type without picture ID using NACK+ULPFEC is a waste of bandwidth since ULPFEC packets also have to be retransmitted. Disabling ULPFEC.
[9270:775:0717/074313.737105:1867358949160:INFO:CONSOLE(606)] "Receiving remote stream...", source: http://127.0.0.1:52903/media/peerconnection-call.html (606)
[9273:25607:0717/074313.879021:1867500862120:WARNING:rtp_frame_reference_finder.cc(207)] Generic frame with packet range [26469, 26470] has no GoP, dropping frame.
[9273:25607:0717/074313.879065:1867500902610:WARNING:rtp_frame_reference_finder.cc(207)] Generic frame with packet range [26465, 26465] has no GoP, dropping frame.
[9273:25607:0717/074313.879083:1867500921804:WARNING:rtp_frame_reference_finder.cc(207)] Generic frame with packet range [26466, 26466] has no GoP, dropping frame.
[9273:25607:0717/074313.879111:1867500949462:WARNING:rtp_frame_reference_finder.cc(207)] Generic frame with packet range [26468, 26468] has no GoP, dropping frame.
[9271:775:0717/074313.911539:1867533397622:ERROR:vt_video_encode_accelerator_mac.cc(495)]  VTCompressionSessionCreate failed: -12915
[9271:775:0717/074313.911721:1867533562198:ERROR:vt_video_encode_accelerator_mac.cc(154)] Failed creating compression session.
[9271:775:0717/074313.911762:1867533602259:ERROR:gpu_video_encode_accelerator_factory.cc(114)] VEA initialize failed
[9271:775:0717/074313.912116:1867533957888:ERROR:gpu_video_encode_accelerator.cc(153)] Initialize Could not create VEA
[9273:29699:0717/074313.912324:1867534166689:ERROR:gpu_video_encode_accelerator_host.cc(109)] Send(GpuCommandBufferMsg_CreateVideoEncoder()) failed
[9273:29699:0717/074313.912374:1867534212952:ERROR:rtc_video_encoder.cc(598)] CreateAndInitializeVEA@../../content/renderer/media/gpu/rtc_video_encoder.cc:341kInvalidArgumentError - Error initializing video_encoder
[9271:17923:0717/074313.912493:1867534334061:ERROR:gpu_channel.cc(726)] Invalid route
[9273:51971:0717/074313.913509:1867535350067:FATAL:ref_counted.h(73)] Check failed: CalledOnValidSequence(). 
0   Content Shell Framework             0x00000001037fca8c base::debug::StackTrace::StackTrace(unsigned long) + 28
1   Content Shell Framework             0x0000000103820810 logging::LogMessage::~LogMessage() + 224
2   Content Shell Framework             0x0000000101a35f0b base::subtle::RefCountedBase::AddRef() const + 299
3   Content Shell Framework             0x0000000107aeb84a base::internal::BindState<void (content::WebRtcVideoCapturerAdapter::TextureFrameCopier::*)(scoped_refptr<media::VideoFrame> const&, scoped_refptr<media::VideoFrame>*, base::WaitableEvent*), scoped_refptr<content::WebRtcVideoCapturerAdapter::TextureFrameCopier>, scoped_refptr<media::VideoFrame>, scoped_refptr<media::VideoFrame>*, base::WaitableEvent*>::BindState<void (content::WebRtcVideoCapturerAdapter::TextureFrameCopier::*)(scoped_refptr<media::VideoFrame> const&, scoped_refptr<media::VideoFrame>*, base::WaitableEvent*), content::WebRtcVideoCapturerAdapter::TextureFrameCopier*, scoped_refptr<media::VideoFrame> const&, scoped_refptr<media::VideoFrame>*&, base::WaitableEvent*>(std::__1::integral_constant<bool, false>, void (*)(), void (content::WebRtcVideoCapturerAdapter::TextureFrameCopier::*&&)(scoped_refptr<media::VideoFrame> const&, scoped_refptr<media::VideoFrame>*, base::WaitableEvent*), content::WebRtcVideoCapturerAdapter::TextureFrameCopier*&&, scoped_refptr<media::VideoFrame> const&&&, scoped_refptr<media::VideoFrame>*&&&, base::WaitableEvent*&&) + 74
4   Content Shell Framework             0x0000000107aea896 content::WebRtcVideoCapturerAdapter::TextureFrameCopier::CopyTextureFrame(scoped_refptr<media::VideoFrame> const&, scoped_refptr<media::VideoFrame>*) + 214
5   Content Shell Framework             0x0000000107aebc7a content::WebRtcVideoFrameAdapter::ToI420() + 90
6   Content Shell Framework             0x00000001057dea1d webrtc::vcm::VideoSender::AddVideoFrame(webrtc::VideoFrame const&, webrtc::CodecSpecificInfo const*) + 717
7   Content Shell Framework             0x00000001057ae6d9 webrtc::ViEEncoder::EncodeVideoFrame(webrtc::VideoFrame const&, long long) + 1609
8   Content Shell Framework             0x00000001057b1e07 webrtc::ViEEncoder::EncodeTask::Run() + 503
9   Content Shell Framework             0x00000001052d9e52 rtc::(anonymous namespace)::RunTask(std::__1::unique_ptr<rtc::QueuedTask, std::__1::default_delete<rtc::QueuedTask> >) + 18
10  Content Shell Framework             0x00000001052da415 base::internal::Invoker<base::internal::BindState<void (*)(std::__1::unique_ptr<rtc::QueuedTask, std::__1::default_delete<rtc::QueuedTask> >), base::internal::PassedWrapper<std::__1::unique_ptr<rtc::QueuedTask, std::__1::default_delete<rtc::QueuedTask> > > >, void ()>::Run(base::internal::BindStateBase*) + 101
11  Content Shell Framework             0x00000001037fd3cb base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask*) + 251
12  Content Shell Framework             0x0000000103839105 base::MessageLoop::RunTask(base::PendingTask*) + 421
13  Content Shell Framework             0x0000000103839545 base::MessageLoop::DeferOrRunPendingTask(base::PendingTask) + 213
14  Content Shell Framework             0x00000001038398a9 base::MessageLoop::DoWork() + 425
15  Content Shell Framework             0x000000010383e30a base::MessagePumpCFRunLoopBase::RunWork() + 42
16  Content Shell Framework             0x00000001038222fa base::mac::CallWithEHFrame(void () block_pointer) + 10
17  Content Shell Framework             0x000000010383dc2f base::MessagePumpCFRunLoopBase::RunWorkSource(void*) + 63
18  CoreFoundation                      0x00007fffc56653b1 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
19  CoreFoundation                      0x00007fffc564663c __CFRunLoopDoSources0 + 556
20  CoreFoundation                      0x00007fffc5645b26 __CFRunLoopRun + 934
21  CoreFoundation                      0x00007fffc5645524 CFRunLoopRunSpecific + 420
22  Content Shell Framework             0x000000010383e75f base::MessagePumpCFRunLoop::DoRun(base::MessagePump::Delegate*) + 79
23  Content Shell Framework             0x000000010383d47c base::MessagePumpCFRunLoopBase::Run(base::MessagePump::Delegate*) + 92
24  Content Shell Framework             0x0000000103838ceb base::MessageLoop::Run() + 219
25  Content Shell Framework             0x000000010386e458 base::RunLoop::Run() + 168
26  Content Shell Framework             0x00000001038b4ece base::Thread::Run(base::RunLoop*) + 206
27  Content Shell Framework             0x00000001038b548a base::Thread::ThreadMain() + 906
28  Content Shell Framework             0x00000001038aa6ef base::(anonymous namespace)::ThreadFunc(void*) + 95
29  libsystem_pthread.dylib             0x00007fffdb4059af _pthread_body + 180
30  libsystem_pthread.dylib             0x00007fffdb4058fb _pthread_body + 0
31  libsystem_pthread.dylib             0x00007fffdb405101 thread_start + 13