Not sure if this is a separate issue from issue 877205...

I don't have much context/familiarity with the test-results url you linked, but if you go to the build, click on one of the "shard" links for the failed site_per_process_webkit_layout_tests test, you can see what appears to be symbolized stack traces.

eg: https://chromium-swarm.appspot.com/task?id=40a336f8c803a510 has a bunch of

20:46:22.478 13008   [15130:15139:1018/204622.262706:FATAL:session_storage_namespace_impl_mojo.cc(136)] Check failed: ::content::BrowserThread::CurrentlyOn(BrowserThread::UI). Must be called on Chrome_UIThread; actually called on Chrome_IOThread.
20:46:22.478 13008   #0 0x0000041e962f base::debug::StackTrace::StackTrace()
20:46:22.478 13008   #1 0x00000414027b logging::LogMessage::~LogMessage()
20:46:22.478 13008   #2 0x000003980e3d content::SessionStorageNamespaceImplMojo::OpenArea()
20:46:22.478 13008   #3 0x00000219c30f blink::mojom::SessionStorageNamespaceStubDispatch::Accept()
20:46:22.478 13008   #4 0x000004228ca2 mojo::InterfaceEndpointClient::HandleValidatedMessage()
20:46:22.478 13008   #5 0x000004228826 mojo::FilterChain::Accept()
20:46:22.478 13008   #6 0x00000422a1a2 mojo::InterfaceEndpointClient::HandleIncomingMessage()
20:46:22.478 13008   #7 0x00000423170c mojo::internal::MultiplexRouter::ProcessIncomingMessage()
20:46:22.478 13008   #8 0x000004230ad0 mojo::internal::MultiplexRouter::Accept()
20:46:22.478 13008   #9 0x000004228826 mojo::FilterChain::Accept()
20:46:22.478 13008   #10 0x000004226003 mojo::Connector::ReadSingleMessage()
20:46:22.478 13008   #11 0x000004226bf4 mojo::Connector::ReadAllAvailableMessages()
20:46:22.478 13008   #12 0x000004226a56 mojo::Connector::OnHandleReadyInternal()
20:46:22.478 13008   #13 0x0000022ccc54 mojo::SimpleWatcher::DiscardReadyState()
20:46:22.478 13008   #14 0x00000421a9a3 mojo::SimpleWatcher::OnHandleReady()
20:46:22.478 13008   #15 0x0000024bba8c _ZN4base8internal7InvokerINS0_9BindStateIMN3viz14GpuServiceImplEFvN3gfx21GenericSharedMemoryIdEiRKN3gpu9SyncTokenEEJNS_7WeakPtrIS4_EES6_iS8_EEEFvvEE7RunImplISC_NSt3__15tupleIJSE_S6_iS8_EEEJLm0ELm1ELm2ELm3EEEEvOT_OT0_NSJ_16integer_sequenceImJXspT1_EEEE
20:46:22.478 13008   #16 0x000004149792 base::debug::TaskAnnotator::RunTask()
20:46:22.478 13008   #17 0x0000041486df base::MessageLoop::RunTask()
20:46:22.478 13008   #18 0x000004148ab2 base::MessageLoop::DoWork()
20:46:22.478 13008   #19 0x000004209f69 base::MessagePumpLibevent::Run()
20:46:22.478 13008   #20 0x0000041481b1 base::MessageLoop::Run()
20:46:22.478 13008   #21 0x00000416db76 base::RunLoop::Run()
20:46:22.478 13008   #22 0x0000041baf4a base::Thread::Run()
20:46:22.478 13008   #23 0x000003866684 content::BrowserProcessSubThread::IOThreadRun()
20:46:22.478 13008   #24 0x0000038665cf content::BrowserProcessSubThread::Run()
20:46:22.478 13008   #25 0x0000041bb2c5 base::Thread::ThreadMain()
20:46:22.478 13008   #26 0x0000041fc98f base::(anonymous namespace)::ThreadFunc()
20:46:22.478 13008   #27 0x7fe05fe88184 start_thread
20:46:22.479 13008   #28 0x7fe05b43903d clone

Does that help at all?

RE: #c3

Thanks - the callstack you posted does help!

RE: I don't have much context/familiarity with the test-results url linked

This was "archive results for webkit_layout_tests (with patch)" step (#114) and "layout_test_results" link.  I think this is generated while running layout tests?

Ahh I see. What if you:

- open up https://test-results.appspot.com/data/layout_results/linux_chromium_rel_ng/213415/site_per_process_webkit_layout_tests%20%28with%20patch%29/layout-test-results/results.html
- click on one of the failed tests
- click on the "stderr" link

It looks like the crash is unintuitively in the "stderr" tab, not the "crash log" tab. I guess you just have to know where to look.