LSC: Replace FALLTHROUGH macro with C++17 [[fallthrough]] attribute

See go/chromium-lsc-fallthrough

This CL was uploaded by git cl split.

R=afakhry@chromium.org

Bug: 1283907
Change-Id: Ibcc05570b751b26156383c4c95f1f772e2795149
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3366199
Auto-Submit: Roland Bock <rbock@google.com>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#955300}

ash/multi_device_setup/multi_device_notification_presenter.cc

@@ -234,7 +234,7 @@ void MultiDeviceNotificationPresenter::OnNotificationClicked(
     case Status::kExistingUserHostSwitchedNotificationVisible:
       // Clicks on the 'host switched' and 'Chromebook added' notifications have
       // the same effect, i.e. opening the Settings subpage.
-      FALLTHROUGH;
+      [[fallthrough]];
     case Status::kExistingUserNewChromebookNotificationVisible:
       Shell::Get()
           ->system_tray_model()

ash/rotator/screen_rotation_animator.cc

@@ -492,7 +492,7 @@ void ScreenRotationAnimator::Rotate(
   switch (screen_rotation_state_) {
     case IDLE:
       DCHECK(!current_async_rotation_request_);
-      FALLTHROUGH;
+      [[fallthrough]];
     case COPY_REQUESTED:
       if (current_async_rotation_request_ &&
           !RootWindowChangedForDisplayId(