The mojo/* LayoutTests fail flakily with the following DCHECK:

crash log for renderer (pid <unknown>):
STDOUT: #CRASHED - renderer
STDERR: [1:12:0626/120604.211560:4138392181665:FATAL:MojoWatcher.cpp(47)] Check failed: !handle_.is_valid(). 
STDERR: #0 0x7fc9e12fc3d7 base::debug::StackTrace::StackTrace()
STDERR: #1 0x7fc9e1321edd logging::LogMessage::~LogMessage()
STDERR: #2 0x7fc9ddbebd69 blink::MojoWatcher::~MojoWatcher()
STDERR: #3 0x7fc9dc57cea6 blink::NormalPage::Sweep()
STDERR: #4 0x7fc9dc578dd3 blink::BaseArena::SweepUnsweptPage()
STDERR: #5 0x7fc9dc579188 blink::BaseArena::CompleteSweep()
STDERR: #6 0x7fc9dc581a31 blink::ThreadState::CompleteSweep()
STDERR: #7 0x7fc9dc58828a blink::ThreadState::PreSweep()
STDERR: #8 0x7fc9dc582584 blink::ThreadState::CollectGarbage()
STDERR: #9 0x7fc9dc581643 blink::ThreadState::RunTerminationGC()
STDERR: #10 0x7fc9dc58150b blink::ThreadState::DetachCurrentThread()
STDERR: #11 0x7fc9dddeaed5 blink::WorkerBackingThread::Shutdown()
STDERR: #12 0x7fc9dddfc2c0 blink::WorkerThread::PerformShutdownOnWorkerThread()
STDERR: #13 0x7fc9dc2c0e84 blink::(anonymous namespace)::RunCrossThreadClosure()
STDERR: #14 0x7fc9dc2c1817 _ZN4base8internal7InvokerINS0_9BindStateIPFvSt10unique_ptrIN3WTF8FunctionIFvvELNS4_22FunctionThreadAffinityE0EEESt14default_deleteIS8_EEEJNS0_13PassedWrapperISB_EEEEES6_E3RunEPNS0_13BindStateBaseE
STDERR: #15 0x7fc9e12e7501 _ZNO4base8CallbackIFvvELNS_8internal8CopyModeE0ELNS2_10RepeatModeE0EE3RunEv
STDERR: #16 0x7fc9e12fccf3 base::debug::TaskAnnotator::RunTask()
STDERR: #17 0x7fc9dc4a4823 blink::scheduler::TaskQueueManager::ProcessTaskFromWorkQueue()
STDERR: #18 0x7fc9dc4a1ee8 blink::scheduler::TaskQueueManager::DoWork()
STDERR: #19 0x7fc9dc4a6c07 _ZN4base8internal7InvokerINS0_9BindStateIMN5blink9scheduler16TaskQueueManagerEFvbEJNS_7WeakPtrIS5_EEbEEEFvvEE3RunEPNS0_13BindStateBaseE
STDERR: #20 0x7fc9e12e7501 _ZNO4base8CallbackIFvvELNS_8internal8CopyModeE0ELNS2_10RepeatModeE0EE3RunEv
STDERR: #21 0x7fc9e12fccf3 base::debug::TaskAnnotator::RunTask()
STDERR: #22 0x7fc9e132f5fd base::MessageLoop::RunTask()
STDERR: #23 0x7fc9e132fa28 base::MessageLoop::DeferOrRunPendingTask()
STDERR: #24 0x7fc9e132fe77 base::MessageLoop::DoWork()
STDERR: #25 0x7fc9e1331869 base::MessagePumpDefault::Run()
STDERR: #26 0x7fc9e132f1d2 base::MessageLoop::Run()
STDERR: #27 0x7fc9e13655b7 base::RunLoop::Run()
STDERR: #28 0x7fc9e13a474c base::Thread::Run()
STDERR: #29 0x7fc9e13a4c6e base::Thread::ThreadMain()
STDERR: #30 0x7fc9e139bc7c base::(anonymous namespace)::ThreadFunc()
STDERR: #31 0x7fc9e32b3184 start_thread
STDERR: #32 0x7fc9da11cbed clone
STDERR: 

I discussed this DCHECK with yzshen@ in another context and the correct answer may be to simply remove it.