The crash seems to be pointing at oilpan attempting to trace the reachable objects on an instance of CompositorProxyClientImpl after part of it has gone away. It seems to be flaky as the bot is not regularly failing.

Crash output from https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Win7__dbg_/5885/layout-test-results/virtual/threaded/fast/compositorworker/compositor-proxy-disconnect-worker-terminate-crash-log.txt follows:

crash log for renderer (pid <unknown>):
STDOUT: #CRASHED - renderer
STDERR: ASSERTION FAILED: header->checkHeader()
STDERR: Backtrace:
STDERR: 	blink::HeapObjectHeader::fromPayload [0x08A0A641+65]
STDERR: 	blink::ObjectAliveTrait<blink::CompositorProxyClientImpl,0>::isHeapObjectAlive [0x0B038D6D+13]
STDERR: 	blink::ThreadHeap::isHeapObjectAlive<blink::CompositorProxyClientImpl> [0x0B035AE1+97]
STDERR: 	blink::ThreadHeap::isHeapObjectAlive<blink::CompositorProxyClientImpl> [0x0B035A71+17]
STDERR: 	WTF::HashTraits<blink::WeakMember<blink::CompositorProxyClientImpl> >::traceInCollection<blink::Visitor *> [0x0B03695B+43]
STDERR: 	WTF::TraceInCollectionTrait<1,1,blink::WeakMember<blink::CompositorProxyClientImpl>,WTF::HashTraits<blink::WeakMember<blink::CompositorProxyClientImpl> > >::trace<blink::Visitor *> [0x0B0363E2+18]
STDERR: 	WTF::WeakProcessingHashTableHelper<1,blink::WeakMember<blink::CompositorProxyClientImpl>,blink::WeakMember<blink::CompositorProxyClientImpl>,WTF::IdentityExtractor,WTF::WeakMemberHash<blink::CompositorProxyClientImpl>,WTF::HashTraits<blink::WeakMember<bli [0x0B0390E0+96]
STDERR: 	blink::CallbackStack::Item::call [0x08F490D9+25]
STDERR: 	blink::ThreadState::popAndInvokeThreadLocalWeakCallback [0x08F64F60+96]
STDERR: 	blink::ThreadState::threadLocalWeakProcessing [0x08F698EC+332]
STDERR: 	blink::ThreadState::preSweep [0x08F65FA1+97]
STDERR: 	blink::ThreadState::safePoint [0x08F67065+181]
STDERR: 	blink::GCTaskObserver::didProcessTask [0x08A7A24A+74]
STDERR: 	scheduler::WebThreadBase::TaskObserverAdapter::DidProcessTask [0x07C2741A+26]
STDERR: 	base::MessageLoop::RunTask [0x02989899+713]
STDERR: 	base::MessageLoop::DeferOrRunPendingTask [0x0298772D+45]
STDERR: 	base::MessageLoop::DoWork [0x02987D14+196]
STDERR: 	base::MessagePumpDefault::Run [0x0298F4AE+190]
STDERR: 	base::MessageLoop::RunHandler [0x02989591+193]
STDERR: 	base::RunLoop::Run [0x02A2E7A4+52]
STDERR: 	base::MessageLoop::Run [0x0298948C+188]
STDERR: 	base::Thread::Run [0x02AB466F+15]
STDERR: 	base::Thread::ThreadMain [0x02AB5119+665]
STDERR: 	base::PlatformThread::Sleep [0x02A97B8C+364]
STDERR: 	BaseThreadInitThunk [0x7654337A+18]
STDERR: 	RtlInitializeExceptionChain [0x772C92B2+99]
STDERR: 	RtlInitializeExceptionChain [0x772C9285+54]