Stop hiding UnifiedSystemTray on last notification

UnifiedSystemTray should not be hidden on the last notification removal,
which is different behavior from old NotificationTray.

TEST=manual
BUG=857336

Change-Id: I083d48455be19d591833a17c9514ca1dbfe6903b
Reviewed-on: https://chromium-review.googlesource.com/1124261Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572525}

ash/system/unified/unified_system_tray.cc

@@ -61,6 +61,7 @@ class UnifiedSystemTray::UiDelegate : public message_center::UiDelegate {
 UnifiedSystemTray::UiDelegate::UiDelegate(UnifiedSystemTray* owner)
     : owner_(owner) {
   ui_controller_ = std::make_unique<message_center::UiController>(this);
+  ui_controller_->set_hide_on_last_notification(false);
   popup_alignment_delegate_ =
       std::make_unique<AshPopupAlignmentDelegate>(owner->shelf());
   message_popup_collection_ =

ui/message_center/ui_controller.cc

@@ -157,7 +157,8 @@ void UiController::OnBlockingStateChanged(NotificationBlocker* blocker) {
 }
 
 void UiController::OnMessageCenterChanged() {
-  if (message_center_visible_ && message_center_->NotificationCount() == 0) {
+  if (hide_on_last_notification_ && message_center_visible_ &&
+      message_center_->NotificationCount() == 0) {
     HideMessageCenterBubble();
     return;
   }

ui/message_center/ui_controller.h

@@ -54,6 +54,9 @@ class MESSAGE_CENTER_EXPORT UiController : public MessageCenterObserver {
   UiDelegate* delegate() { return delegate_; }
   const MessageCenter* message_center() const { return message_center_; }
   MessageCenter* message_center() { return message_center_; }
+  void set_hide_on_last_notification(bool hide_on_last_notification) {
+    hide_on_last_notification_ = hide_on_last_notification;
+  }
 
   // Overridden from MessageCenterObserver:
   void OnNotificationAdded(const std::string& notification_id) override;
@@ -80,6 +83,9 @@ class MESSAGE_CENTER_EXPORT UiController : public MessageCenterObserver {
   bool popups_visible_ = false;
   UiDelegate* delegate_;
 
+  // Set true to hide MessageCenterView when the last notification is dismissed.
+  bool hide_on_last_notification_ = true;
+
   DISALLOW_COPY_AND_ASSIGN(UiController);
 };