ash: Use CONTINUE_ON_SHUTDOWN for AccelerometerReader

AccelerometerReader's data reading blocks shutdown on certain
devices. Use CONTINUE_ON_SHUTDOWN to run it on a thread that
Chrome does not wait to join during shutdown.

Bug: 1023989
Change-Id: I4fdad845fd59b45f3dc58a28edea031e733d934a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916691Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715336}

ash/shell.cc

@@ -546,11 +546,13 @@ Shell::Shell(std::unique_ptr<ShellDelegate> shell_delegate,
 
   // AccelerometerReader is important for screen orientation so we need
   // USER_VISIBLE priority.
+  // Use CONTINUE_ON_SHUTDOWN to avoid blocking shutdown since the data reading
+  // could get blocked on certain devices. See https://crbug.com/1023989.
   AccelerometerReader::GetInstance()->Initialize(
       base::CreateSequencedTaskRunner(
           {base::ThreadPool(), base::MayBlock(),
            base::TaskPriority::USER_VISIBLE,
-           base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN}));
+           base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN}));
 
   login_screen_controller_ =
       std::make_unique<LoginScreenController>(system_tray_notifier_.get());