Chrome OS crash reading unregistered Docked Magnifier preference.

On ToT @ #549307 build chrome os on linux desktop.
(1) Run chrome, open settings from system tray or about:settings
(2) Enable the "Always show accessibility options in the system menu" option.
(3) Click the system tray to show the bubble.
Expected: Bubble shows, no crash.
Actual: Crash; stack below.

Ahmed, please take a look, it appears that registration occurs here:
https://cs.chromium.org/chromium/src/ash/magnifier/docked_magnifier_controller.cc?rcl=c2bbed2e92b8ec200bb6d83056088c810366cc49&l=148

[75936:75936:0410/111422.492304:FATAL:pref_service.cc(121)] Check failed: false. Trying to read an unregistered pref: ash.docked_magnifier.enabled
#0 0x7ffb7f9bdf4c base::debug::StackTrace::StackTrace()
#1 0x7ffb7f9e4e2b logging::LogMessage::~LogMessage()
#2 0x7ffb7ae93062 PrefService::GetBoolean()
#3 0x5561e305f10d chromeos::AccessibilityManager::ShouldShowAccessibilityMenu()
#4 0x5561e497a3a3 (anonymous namespace)::AccessibilityDelegateImpl::ShouldShowAccessibilityMenu()
#5 0x7ffb79c432f6 ash::TrayAccessibility::CreateDefaultView()
#6 0x7ffb79c38584 ash::SystemTrayView::CreateItemViews()
#7 0x7ffb79c35059 ash::SystemTrayBubble::InitView()
#8 0x7ffb79c333ab ash::SystemBubbleWrapper::InitView()
#9 0x7ffb79c329df ash::SystemTray::ShowItems()
#10 0x7ffb79c327f9 ash::SystemTray::ShowDefaultView()
#11 0x7ffb79c33871 ash::SystemTray::PerformAction()
#12 0x7ffb79c2e209 ash::ActionableView::ButtonPressed()
#13 0x7ffb7a1ea21d views::Button::OnMouseReleased()
#14 0x7ffb7a1d8669 views::InkDropHostView::OnMouseEvent()
#15 0x7ffb7b5b9482 ui::ScopedTargetHandler::OnEvent()
#16 0x7ffb7b5b6174 ui::EventDispatcher::ProcessEvent()
#17 0x7ffb7b5b5f13 ui::EventDispatcherDelegate::DispatchEvent()
#18 0x7ffb7a25d08d views::internal::RootView::OnMouseReleased()
#19 0x7ffb7a262c67 views::Widget::OnMouseEvent()
#20 0x7ffb7a281526 views::NativeWidgetAura::OnMouseEvent()
#21 0x7ffb7b5b6174 ui::EventDispatcher::ProcessEvent()
#22 0x7ffb7b5b5f13 ui::EventDispatcherDelegate::DispatchEvent()
#23 0x7ffb7b5b7712 ui::EventProcessor::OnEventFromSource()
#24 0x7ffb7b5b7d51 ui::EventSource::SendEventToSink()
#25 0x7ffb7b31a6ca aura::WindowTreeClient::OnWindowInputEvent()
#26 0x7ffb7b35fe86 ui::mojom::WindowTreeClientStubDispatch::Accept()
#27 0x7ffb7ec6caa6 mojo::InterfaceEndpointClient::HandleValidatedMessage()
#28 0x7ffb7ec6c3e6 mojo::FilterChain::Accept()
#29 0x7ffb7ec6ddd5 mojo::InterfaceEndpointClient::HandleIncomingMessage()
#30 0x7ffb7ec7453c mojo::internal::MultiplexRouter::ProcessIncomingMessage()
#31 0x7ffb7ec73940 mojo::internal::MultiplexRouter::Accept()
#32 0x7ffb7ec6c3e6 mojo::FilterChain::Accept()
#33 0x7ffb7ec675c3 mojo::Connector::ReadSingleMessage()
#34 0x7ffb7ec67ff1 mojo::Connector::ReadAllAvailableMessages()
#35 0x7ffb7ec67e99 mojo::Connector::OnHandleReadyInternal()
#36 0x7ffb7ec68727 mojo::SimpleWatcher::DiscardReadyState()
#37 0x7ffb7ec30f26 mojo::SimpleWatcher::OnHandleReady()
#38 0x7ffb7ec31431 _ZN4base8internal7InvokerINS0_9BindStateIMN4mojo13SimpleWatcherEFvijRKNS3_18HandleSignalsStateEEJNS_7WeakPtrIS4_EEijS5_EEEFvvEE7RunImplIRKS9_RKNSt3__15tupleIJSB_ijS5_EEEJLm0ELm1ELm2ELm3EEEEvOT_OT0_NSI_16integer_sequenceImJXspT1_EEEE
#39 0x7ffb7f9be8e5 base::debug::TaskAnnotator::RunTask()
#40 0x7ffb7f9f1099 base::internal::IncomingTaskQueue::RunTask()
#41 0x7ffb7f9f4bdb base::MessageLoop::RunTask()
#42 0x7ffb7f9f4f6a base::MessageLoop::DeferOrRunPendingTask()
#43 0x7ffb7f9f51cc base::MessageLoop::DoWork()
#44 0x7ffb7f9f75f9 base::MessagePumpLibevent::Run()
#45 0x7ffb7f9f4509 base::MessageLoop::Run()
#46 0x7ffb7fa2b209 base::RunLoop::Run()
#47 0x5561e355e75a ChromeBrowserMainParts::MainMessageLoopRun()
#48 0x7ffb7cbd8b67 content::BrowserMainLoop::RunMainMessageLoopParts()
#49 0x7ffb7cbdbc66 content::BrowserMainRunnerImpl::Run()
#50 0x7ffb7cbd4df9 content::BrowserMain()
#51 0x7ffb7d5adf7d content::ContentMainRunnerImpl::Run()
#52 0x7ffb7ff03322 service_manager::Main()
#53 0x7ffb7d5ac5f4 content::ContentMain()
#54 0x5561e29d9133 ChromeMain
#55 0x7ffb7243f2b1 __libc_start_main
#56 0x5561e29d8faa _start