dark_mode: Add down arrow to dark theme feature pod

Bug: 1164980
Change-Id: Iadde28d35b316f811b2d60637dd27addc4525132
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2621747Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842292}

ash/system/dark_mode/dark_mode_feature_pod_controller.cc

@@ -33,6 +33,7 @@ FeaturePodButton* DarkModeFeaturePodController::CreateButton() {
   button_->SetLabel(l10n_util::GetStringUTF16(IDS_ASH_STATUS_TRAY_DARK_THEME));
   button_->SetLabelTooltip(l10n_util::GetStringUTF16(
       IDS_ASH_STATUS_TRAY_DARK_THEME_SETTINGS_TOOLTIP));
+  button_->ShowDetailedViewArrow();
   // TODO(minch): Add the logic for login screen.
   button_->SetVisible(
       Shell::Get()->session_controller()->IsActiveUserSessionStarted());