CL: https://chromium-review.googlesource.com/c/chromium/src/+/1382667/10

Notice that linux_layout_tests_layout_ng failed 3 times, and then finally succeeded [and CL was landed] on 4th try. 

Looking at an example task from the failing build: 
https://chromium-swarm.appspot.com/task?id=41e38f7435907f10&refresh=10&show_raw=1

"""

00:33:48.622 2670   [1:1:1220/003348.302685:FATAL:ng_caret_position.cc(270)] Check failed: candidate.IsNull(). 
00:33:48.622 2670   #0 0x5651c363d4ff base::debug::StackTrace::StackTrace()
00:33:48.622 2670   #1 0x5651c358d1ca logging::LogMessage::~LogMessage()
00:33:48.622 2670   #2 0x5651c5eb6c65 blink::ComputeNGCaretPosition()
00:33:48.622 2670   #3 0x5651c5eb793d blink::ComputeNGCaretPosition()
00:33:48.622 2670   #4 0x5651c5eb8197 blink::ComputeNGLocalCaretRect()
00:33:48.622 2670   #5 0x5651c57b2e7b blink::LocalCaretRectOfPosition()
00:33:48.622 2670   #6 0x5651c57b348d blink::AbsoluteCaretRectOfPosition()
00:33:48.622 2670   #7 0x5651c5819131 blink::FirstRectForRange()
00:33:48.622 2670   #8 0x5651c579018b blink::FrameSelection::ComputeAbsoluteBounds()
00:33:48.622 2670   #9 0x5651c5853bbc blink::WebViewImpl::SelectionBounds()
00:33:48.622 2670   #10 0x5651c6bad640 blink::WebViewFrameWidget::SelectionBounds()
00:33:48.622 2670   #11 0x5651c73069c5 content::RenderWidget::GetSelectionBounds()
00:33:48.622 2670   #12 0x5651c7300be0 content::RenderWidget::UpdateSelectionBounds()
00:33:48.622 2670   #13 0x5651c7300920 content::RenderWidget::WillBeginCompositorFrame()
00:33:48.622 2670   #14 0x5651c4862094 cc::SingleThreadProxy::DoBeginMainFrame()
00:33:48.622 2670   #15 0x5651c4861e04 cc::SingleThreadProxy::CompositeImmediately()
00:33:48.622 2670   #16 0x5651c47a968a cc::LayerTreeHost::Composite()
00:33:48.622 2670   #17 0x5651c73f9eff content::LayerTreeView::SynchronouslyComposite()
00:33:48.622 2670   #18 0x5651c73fb6f9 _ZN4base8internal7InvokerINS0_9BindStateIMN7content13LayerTreeViewEFvbNSt3__110unique_ptrIN2cc11SwapPromiseENS5_14default_deleteIS8_EEEEEJNS_7WeakPtrIS4_EEbDnEEEFvvEE7RunImplISD_NS5_5tupleIJSF_bDnEEEJLm0ELm1ELm2EEEEvOT_OT0_NS5_16integer_sequenceImJXspT1_EEEE
00:33:48.622 2670   #19 0x5651c3597d49 base::debug::TaskAnnotator::RunTask()
00:33:48.622 2670   #20 0x5651c35e5edf base::sequence_manager::internal::ThreadControllerImpl::DoWork()
00:33:48.622 2670   #21 0x5651c0f9b208 _ZN4base8internal7InvokerINS0_9BindStateIMN3net14MDnsConnectionEFviEJNS_7WeakPtrIS4_EEiEEEFvvEE3RunEPNS0_13BindStateBaseE
00:33:48.622 2670   #22 0x5651c3597d49 base::debug::TaskAnnotator::RunTask()
00:33:48.623 2670   #23 0x5651c3597000 base::MessageLoopImpl::RunTask()
00:33:48.623 2670   #24 0x5651c35976f2 base::MessageLoopImpl::DoWork()
00:33:48.623 2670   #25 0x5651c3599a7a base::MessagePumpDefault::Run()
00:33:48.623 2670   #26 0x5651c3596ad5 base::MessageLoopImpl::Run()
00:33:48.623 2670   #27 0x5651c35bbe26 base::RunLoop::Run()
00:33:48.623 2670   #28 0x5651c7341c84 content::RendererMain()
00:33:48.623 2670   #29 0x5651c2b84233 content::RunZygote()
00:33:48.623 2670   #30 0x5651c2b8555e content::ContentMainRunnerImpl::Run()
00:33:48.623 2670   #31 0x5651c4db87c0 service_manager::Main()
00:33:48.623 2670   #32 0x5651c180bdb1 content::ContentMain()
00:33:48.623 2670   #33 0x5651c0ae616b main
00:33:48.623 2670   #34 0x7f240dd48f45 __libc_start_main
00:33:48.623 2670   #35 0x5651c0ae602a _start
"""

It looks like there are real CHECKs being hit.