mash: WeakPtr DCHECK crash on device startup from OzonePlatformGbm

On Chrome OS ToT@#539689 (66.0.3350.3) on cyan (likely elsewhere too)
(1) Run a debug build of chrome with --enable-features=Mash
Expected: Chrome starts up normally
Actual: Crash on startup, callstack below, looks like weak ptr use on the wrong thread.

8837:8848:0228/161851.236491:FATAL:weak_ptr.cc(26)] Check failed: sequence_checker_.CalledOnValidSequence(). WeakPtrs must be checked on the same sequenced thread.
#0 0x609710538a68 base::debug::StackTrace::StackTrace()
#1 0x609710536f8c base::debug::StackTrace::StackTrace()
#2 0x6097105a2f2a logging::LogMessage::~LogMessage()
#3 0x6097105b5b6d base::internal::WeakReference::Flag::IsValid()
#4 0x6097105b5cfe base::internal::WeakReference::is_valid()
#5 0x609708a4666f base::WeakPtr<>::get()
#6 0x609708a46535 base::WeakPtr<>::operator bool()
#7 0x609708a481b9 _ZN4base26CallbackCancellationTraitsIMN2ui12_GLOBAL__N_116OzonePlatformGbmEFvvENSt3__15tupleIJNS_7WeakPtrIS3_EEEEEvE11IsCancelledIS9_JEEEbRKS5_RKT_DpRKT0_
#8 0x609708a4818d _ZN4base8internal27ApplyCancellationTraitsImplIMN2ui12_GLOBAL__N_116OzonePlatformGbmEFvvENSt3__15tupleIJNS_7WeakPtrIS4_EEEEEJLm0EEEEbRKT_RKT0_NS7_16integer_sequenceImJXspT1_EEEE
#9 0x609708a480dc _ZN4base8internal23ApplyCancellationTraitsINS0_9BindStateIMN2ui12_GLOBAL__N_116OzonePlatformGbmEFvvEJNS_7WeakPtrIS5_EEEEEEEbPKNS0_13BindStateBaseE
#10 0x609710510eab base::internal::BindStateBase::IsCancelled()
#11 0x609710510a6e base::internal::CallbackBase::IsCancelled()
#12 0x6097105baa42 base::MessageLoop::DoWork()
#13 0x6097105bcd7a base::MessagePumpDefault::Run()
#14 0x6097105b9d92 base::MessageLoop::Run()
#15 0x609710655070 base::RunLoop::Run()
#16 0x6097106fa4d8 base::Thread::Run()
#17 0x6097106fb4bc base::Thread::ThreadMain()
#18 0x6097106f105d base::(anonymous namespace)::ThreadFunc()
#19 0x726009f752b8 <unknown>
#20 0x7260090b0fad clone

Sadrul, can you take a look? Thanks!