Could be 
- false positive
- missing write barrier
- weak handling that is not safe for incremental marking

Collected using
  is_component_build = true
  is_debug = true
  symbol_level = 2
  enable_blink_heap_verification = true
  enable_blink_heap_incremental_marking = true

Test
  fast/dom/shadow/content-pseudo-element-reprojection.html

crash log for renderer (pid <unknown>):
STDOUT: <empty>
STDERR: [1:1:0403/203703.925186:FATAL:MarkingVerifier.h(69)] Check failed: child_header->IsMarked(). 
STDERR: #0 0x7f46ebb2b55d base::debug::StackTrace::StackTrace()
STDERR: #1 0x7f46ebb29b1c base::debug::StackTrace::StackTrace()
STDERR: #2 0x7f46ebbb05aa logging::LogMessage::~LogMessage()
STDERR: #3 0x7f46da284e45 blink::MarkingVerifier::VerifyChild()
STDERR: #4 0x7f46da284b51 blink::MarkingVerifier::Visit()
STDERR: #5 0x7f46dc20f6ae blink::Visitor::Trace<>()
STDERR: #6 0x7f46dc20f630 blink::Visitor::Trace<>()
STDERR: #7 0x7f46dc210a8e blink::CachedMatchedPropertiesHashTraits::TraceInCollection<>()
STDERR: #8 0x7f46dc2109bf WTF::TraceNoWeakHandlingInCollectionHelper<>::Trace<>()
STDERR: #9 0x7f46dc21098d WTF::TraceInCollectionTrait<>::Trace<>()
STDERR: #10 0x7f46dc21095d blink::TraceCollectionIfEnabled<>::Trace<>()
STDERR: #11 0x7f46dc210921 WTF::TraceInCollectionTrait<>::Trace<>()
STDERR: #12 0x7f46dc2108dd blink::TraceCollectionIfEnabled<>::Trace<>()
STDERR: #13 0x7f46dc21088a WTF::TraceInCollectionTrait<>::Trace<>()
STDERR: #14 0x7f46dc2107ed blink::TraceTrait<>::Trace<>()
STDERR: #15 0x7f46da2846b4 blink::MarkingVerifier::VerifyObject()
STDERR: #16 0x7f46da27da7c blink::NormalPage::VerifyMarking()
STDERR: #17 0x7f46da27d873 blink::NormalPageArena::VerifyMarking()
STDERR: #18 0x7f46da26bc2e blink::ThreadHeap::VerifyMarking()
STDERR: #19 0x7f46da2985c7 blink::ThreadState::VerifyMarking()
STDERR: #20 0x7f46da297f06 blink::ThreadState::MarkPhaseEpilogue()
STDERR: #21 0x7f46da294ddb blink::ThreadState::IncrementalMarkingFinalize()
STDERR: #22 0x7f46da294593 blink::ThreadState::RunScheduledGC()
STDERR: #23 0x7f46da296367 blink::ThreadState::SafePoint()
STDERR: #24 0x7f46d9d23f8e blink::GCTaskObserver::DidProcessTask()
STDERR: #25 0x7f46da397109 blink::scheduler::WebThreadBase::TaskObserverAdapter::DidProcessTask()
STDERR: #26 0x7f46da36a5bf blink::scheduler::TaskQueueManagerImpl::NotifyDidProcessTask()
STDERR: #27 0x7f46da36a04d blink::scheduler::TaskQueueManagerImpl::DidRunTask()
STDERR: #28 0x7f46da37add7 blink::scheduler::internal::ThreadControllerImpl::DoWork()
STDERR: #29 0x7f46da37db21 _ZN4base8internal13FunctorTraitsIMN5blink9scheduler8internal20ThreadControllerImplEFvNS4_19SequencedTaskSource8WorkTypeEEvE6InvokeIRKNS_7WeakPtrIS5_EEJRKS7_EEEvS9_OT_DpOT0_
STDERR: #30 0x7f46da37da85 _ZN4base8internal12InvokeHelperILb1EvE8MakeItSoIRKMN5blink9scheduler8internal20ThreadControllerImplEFvNS6_19SequencedTaskSource8WorkTypeEERKNS_7WeakPtrIS7_EEJRKS9_EEEvOT_OT0_DpOT1_
STDERR: #31 0x7f46da37d9fd _ZN4base8internal7InvokerINS0_9BindStateIMN5blink9scheduler8internal20ThreadControllerImplEFvNS5_19SequencedTaskSource8WorkTypeEEJNS_7WeakPtrIS6_EES8_EEEFvvEE7RunImplIRKSA_RKNSt3__15tupleIJSC_S8_EEEJLm0ELm1EEEEvOT_OT0_NSJ_16integer_sequenceImJXspT1_EEEE
STDERR: #32 0x7f46da37d90c _ZN4base8internal7InvokerINS0_9BindStateIMN5blink9scheduler8internal20ThreadControllerImplEFvNS5_19SequencedTaskSource8WorkTypeEEJNS_7WeakPtrIS6_EES8_EEEFvvEE3RunEPNS0_13BindStateBaseE
STDERR: #33 0x7f46ebada1ae _ZNO4base12OnceCallbackIFvvEE3RunEv
STDERR: #34 0x7f46ebb2f43f base::debug::TaskAnnotator::RunTask()
STDERR: #35 0x7f46ebbcf109 base::internal::IncomingTaskQueue::RunTask()
STDERR: #36 0x7f46ebbd82c5 base::MessageLoop::RunTask()
STDERR: #37 0x7f46ebbd8548 base::MessageLoop::DeferOrRunPendingTask()
STDERR: #38 0x7f46ebbd8879 base::MessageLoop::DoWork()
STDERR: #39 0x7f46ebbda8b7 base::MessagePumpDefault::Run()
STDERR: #40 0x7f46ebbd7a8c base::MessageLoop::Run()
STDERR: #41 0x7f46ebc8ebed base::RunLoop::Run()
STDERR: #42 0x7f46ea14a215 content::RendererMain()
STDERR: #43 0x7f46ea575391 content::RunZygote()
STDERR: #44 0x7f46ea5760de content::RunNamedProcessTypeMain()
STDERR: #45 0x7f46ea578604 content::ContentMainRunnerImpl::Run()
STDERR: #46 0x7f46ea56ead5 content::ContentServiceManagerMainDelegate::RunEmbedderProcess()
STDERR: #47 0x7f46e018fdfc service_manager::Main()
STDERR: #48 0x7f46ea574cd5 content::ContentMain()
STDERR: #49 0x00000061d155 main
STDERR: #50 0x7f46cfb482b1 __libc_start_main
STDERR: #51 0x00000061d02a _start
STDERR: