cros: Fix crash in LockContentsView::FocusNextUser

- Also fixed in FocusPreviousUser
- Add unit test to validate crash does not regress

Bug: 895589
Change-Id: Ibb436d95b558089d36a8b10f33fab2bc88568214
Reviewed-on: https://chromium-review.googlesource.com/c/1320212Reviewed-by: Quan Nguyen <qnnguyen@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605811}

ash/login/ui/lock_contents_view.cc

@@ -418,6 +418,9 @@ LockContentsView::~LockContentsView() {
 }
 
 void LockContentsView::FocusNextUser() {
+  if (users_.empty())
+    return;
+
   if (login_views_utils::HasFocusInAnyChildView(primary_big_view_)) {
     if (opt_secondary_big_view_) {
       SwapActiveAuthBetweenPrimaryAndSecondary(false /*is_primary*/);
@@ -454,6 +457,9 @@ void LockContentsView::FocusNextUser() {
 }
 
 void LockContentsView::FocusPreviousUser() {
+  if (users_.empty())
+    return;
+
   if (login_views_utils::HasFocusInAnyChildView(primary_big_view_)) {
     if (users_list_) {
       users_list_->user_view_at(users_list_->user_count() - 1)->RequestFocus();

ash/login/ui/lock_contents_view_unittest.cc

@@ -2138,4 +2138,17 @@ TEST_F(LockContentsViewUnitTest,
   Shell::Get()->power_button_controller()->OnTabletModeEnded();
 }
 
+TEST_F(LockContentsViewUnitTest, RightAndLeftAcceleratorsWithNoUser) {
+  // Show lock screen but do *not* initialize any users.
+  auto* lock = new LockContentsView(
+      mojom::TrayActionState::kNotAvailable, LockScreen::ScreenType::kLock,
+      data_dispatcher(),
+      std::make_unique<FakeLoginDetachableBaseModel>(data_dispatcher()));
+  SetWidget(CreateWidgetWithContent(lock));
+
+  // Nothing to validate except that there is no crash.
+  lock->AcceleratorPressed(ui::Accelerator(ui::VKEY_RIGHT, 0));
+  lock->AcceleratorPressed(ui::Accelerator(ui::VKEY_LEFT, 0));
+}
+
 }  // namespace ash