Saw this during an engine run today,

[118804:118804:0822/201727:FATAL:router.h(65)] Check failed: thread_checker_.CalledOnValidThread(). 
#0 0x7f8973a2b33e base::debug::StackTrace::StackTrace()
#1 0x7f8973a9189f logging::LogMessage::~LogMessage()
#2 0x0000005bc894 mojo::internal::Router::CloseMessagePipe()
#3 0x0000005bb35f mojo::internal::SimpleBindingState::Close()
#4 0x00000044c7c5 mojo::Binding<>::Close()
#5 0x00000044c185 mojo::BindingSet<>::Element::Close()
#6 0x00000044bd5f mojo::BindingSet<>::CloseAllBindings()
#7 0x00000044b029 mojo::BindingSet<>::~BindingSet()
#8 0x00000044a661 blimp::engine::BlobChannelService::~BlobChannelService()
#9 0x00000044a699 blimp::engine::BlobChannelService::~BlobChannelService()
#10 0x0000004289cf std::default_delete<>::operator()()
#11 0x0000004c599c std::unique_ptr<>::reset()
#12 0x0000004c2d09 std::unique_ptr<>::~unique_ptr()
#13 0x0000004bebfa blimp::engine::BlimpEngineSession::~BlimpEngineSession()
#14 0x0000004bed39 blimp::engine::BlimpEngineSession::~BlimpEngineSession()
#15 0x0000004289cf std::default_delete<>::operator()()
#16 0x00000042822c std::unique_ptr<>::reset()
#17 0x000000427d7d blimp::engine::BlimpBrowserMainParts::PostMainMessageLoopRun()
#18 0x7f89706ed878 content::BrowserMainLoop::ShutdownThreadsAndCleanUp()
#19 0x7f89706f7323 content::BrowserMainRunnerImpl::Shutdown()
#20 0x7f89706e73c5 content::BrowserMain()
#21 0x7f89722b95c6 content::RunNamedProcessTypeMain()
#22 0x7f89722bb8b2 content::ContentMainRunnerImpl::Run()
#23 0x7f89722b8682 content::ContentMain()
#24 0x000000427905 main
#25 0x7f8967410f45 __libc_start_main
#26 0x0000004277d5 <unknown>

Kevin, do you mind taking a look