I just got this while running ToT (4f26aea6b7ea8) in a CrOS-on-Linux build:

[6772:6791:0524/153246.018497:FATAL:pref_service.cc(117)] Check failed: CalledOnValidThread(). 
#0 0x7f470ffde5cc base::debug::StackTrace::StackTrace()
#1 0x7f4710002b21 logging::LogMessage::~LogMessage()
#2 0x7f470e919283 PrefService::GetBoolean()
#3 0x55cb9cde22cc chromeos::AccessibilityManager::UpdateChromeOSAccessibilityHistograms()
#4 0x7f470d5ba5f7 content::BrowserAccessibilityStateImpl::UpdateHistograms()
#5 0x7f470d2a577c _ZN4base8internal7InvokerINS0_9BindStateIMN7content17WebFileSystemImpl23WaitableCallbackResultsEFvvEJ13scoped_refptrIS5_EEEEFvvEE3RunEPNS0_13BindStateBaseE
#6 0x7f470ffc9a99 _ZNO4base8CallbackIFvvELNS_8internal8CopyModeE0ELNS2_10RepeatModeE0EE3RunEv
#7 0x7f470ffdef64 base::debug::TaskAnnotator::RunTask()
#8 0x7f471006fc97 base::internal::TaskTracker::PerformRunTask()
#9 0x7f4710070327 base::internal::TaskTrackerPosix::PerformRunTask()
#10 0x7f471006f487 base::internal::TaskTracker::RunTask()
#11 0x7f4710069c4e base::internal::SchedulerWorker::Thread::ThreadMain()
#12 0x7f471007a0cf base::(anonymous namespace)::ThreadFunc()
#13 0x7f4710157184 start_thread
#14 0x7f47043b3bed clone

Received signal 6
#0 0x7f470ffde5cc base::debug::StackTrace::StackTrace()
#1 0x7f470ffde131 base::debug::(anonymous namespace)::StackDumpSignalHandler()
#2 0x7f471015f330 <unknown>
#3 0x7f47042ecc37 gsignal
#4 0x7f47042f0028 abort
#5 0x7f470ffdc445 base::debug::BreakDebugger()
#6 0x7f4710002ee7 logging::LogMessage::~LogMessage()
#7 0x7f470e919283 PrefService::GetBoolean()
#8 0x55cb9cde22cc chromeos::AccessibilityManager::UpdateChromeOSAccessibilityHistograms()
#9 0x7f470d5ba5f7 content::BrowserAccessibilityStateImpl::UpdateHistograms()
#10 0x7f470d2a577c _ZN4base8internal7InvokerINS0_9BindStateIMN7content17WebFileSystemImpl23WaitableCallbackResultsEFvvEJ13scoped_refptrIS5_EEEEFvvEE3RunEPNS0_13BindStateBaseE
#11 0x7f470ffc9a99 _ZNO4base8CallbackIFvvELNS_8internal8CopyModeE0ELNS2_10RepeatModeE0EE3RunEv
#12 0x7f470ffdef64 base::debug::TaskAnnotator::RunTask()
#13 0x7f471006fc97 base::internal::TaskTracker::PerformRunTask()
#14 0x7f4710070327 base::internal::TaskTrackerPosix::PerformRunTask()
#15 0x7f471006f487 base::internal::TaskTracker::RunTask()
#16 0x7f4710069c4e base::internal::SchedulerWorker::Thread::ThreadMain()
#17 0x7f471007a0cf base::(anonymous namespace)::ThreadFunc()
#18 0x7f4710157184 start_thread
#19 0x7f47043b3bed clone
  r8: ffffae598dbf3608  r9: ffffae598dbf35f8 r10: 0000000000000008 r11: 0000000000000206
 r12: 00007f4704b96940 r13: 00007f46f8fbbe48 r14: 000000000000005f r15: 00007f46f8fbbe40
  di: 0000000000001a74  si: 0000000000001a87  bp: 00007f46f8fbb9a0  bx: 00007f46f8fbb9b0
  dx: 0000000000000006  ax: 0000000000000000  cx: 00007f47042ecc37  sp: 00007f46f8fbb868
  ip: 00007f47042ecc37 efl: 0000000000000206 cgf: 0000000000000033 erf: 0000000000000000
 trp: 0000000000000000 msk: 0000000000000000 cr2: 0000000000000000
[end of stack trace]
Calling _exit(1). Core file will not be generated.