Re-factor system tray code controlling launcher visibility

ash/system/tray/tray_background_view.cc

Index: ash/system/tray/tray_background_view.cc
diff --git a/ash/system/tray/tray_background_view.cc b/ash/system/tray/tray_background_view.cc
index 8b554f7a0af4d627298f00f1eeb38793c3281b6f..c4a4ebbdcac7ae398609bc395e1bae9b29f87541 100644
--- a/ash/system/tray/tray_background_view.cc
+++ b/ash/system/tray/tray_background_view.cc
@@ -187,11 +187,13 @@ void TrayBackgroundView::Initialize() {
 void TrayBackgroundView::OnMouseEntered(const ui::MouseEvent& event) {
   hover_background_animator_.SetPaintsBackground(true,
       internal::BackgroundAnimator::CHANGE_ANIMATE);
+  UpdateShouldShowLauncher();
 }
 
 void TrayBackgroundView::OnMouseExited(const ui::MouseEvent& event) {
   hover_background_animator_.SetPaintsBackground(false,
       internal::BackgroundAnimator::CHANGE_ANIMATE);
+  UpdateShouldShowLauncher();
 }
 
 void TrayBackgroundView::ChildPreferredSizeChanged(views::View* child) {
@@ -268,5 +270,9 @@ void TrayBackgroundView::SetBorder() {
   }
 }
 
+void TrayBackgroundView::UpdateShouldShowLauncher() {
+  status_area_widget()->UpdateShouldShowLauncher();
+}
+
 }  // namespace internal
 }  // namespace ash