M69 manually built and deployed to caroline:

* Shortly after sign-in, click on system tray, then tap the shutdown icon with your finger
* Shutdown crash:

#0 0x59c0d693695c base::debug::StackTrace::StackTrace()
#1 0x59c0d69364c1 base::debug::(anonymous namespace)::StackDumpSignalHandler()
#2 0x736fde7512e0 <unknown>
#3 0x59c0d8b54da8 ash::ArcNotificationManager::~ArcNotificationManager()
#4 0x59c0d8b54fce ash::ArcNotificationManager::~ArcNotificationManager()
#5 0x59c0d8a787a2 ash::MessageCenterController::~MessageCenterController()
#6 0x59c0d8a788be ash::MessageCenterController::~MessageCenterController()
#7 0x59c0d89959c4 ash::Shell::~Shell()
#8 0x59c0d8996e2e ash::Shell::~Shell()
#9 0x59c0d8d689fa ChromeBrowserMainExtraPartsAsh::PostMainMessageLoopRun()
#10 0x59c0d659a25a ChromeBrowserMainParts::PostMainMessageLoopRun()
#11 0x59c0d589a3f6 chromeos::ChromeBrowserMainPartsChromeos::PostMainMessageLoopRun()
#12 0x59c0d4e45354 content::BrowserMainLoop::ShutdownThreadsAndCleanUp()
#13 0x59c0d4e47c03 content::BrowserMainRunnerImpl::Shutdown()
#14 0x59c0d4e415ef content::BrowserMain()
#15 0x59c0d6589952 content::ContentMainRunnerImpl::Run()
#16 0x59c0d65908dc service_manager::Main()
#17 0x59c0d6587921 content::ContentMain()
#18 0x59c0d4308b3f ChromeMain

I might have had open notifications at the time.

I suspect the problem is that ArcNotificationManager holds a raw pointer to the message center.

MessageCenterController::~MessageCenterController() {
  all_popup_blocker_.reset();
  session_state_notification_blocker_.reset();
  inactive_user_notification_blocker_.reset();
  fullscreen_notification_blocker_.reset();
  // *** should delete arc_notification_manager_ here ***

  message_center::MessageCenter::Shutdown();
  // *** actually deleted here ***
}

estade's CL might have caused this, https://chromium-review.googlesource.com/c/chromium/src/+/940186