Ash Tray: Fix tab focus when opening audio settings from volume slider bubble.

BUG=894780

Change-Id: Ib6066153edf992baf4572113155b95d677ac8af7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1714265Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680296}

ash/system/unified/unified_system_tray.cc

@@ -193,7 +193,8 @@ void UnifiedSystemTray::ShowVolumeSliderBubble() {
 }
 
 void UnifiedSystemTray::ShowAudioDetailedViewBubble() {
-  ShowBubble(false /* show_by_click */);
+  // The settings menu bubble gains focus when |show_by_click| is true.
+  ShowBubble(true /* show_by_click */);
   bubble_->ShowAudioDetailedView();
 }