Add early return in message_center::UiController.

This is a follow up to 309d7b01

Bug: 795331
Change-Id: I2765f13bf66e2fd0f1a0cce94c7a4c84fd237c3c
Reviewed-on: https://chromium-review.googlesource.com/923356Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537801}

ui/message_center/ui_controller.cc

@@ -161,8 +161,10 @@ void UiController::OnBlockingStateChanged(NotificationBlocker* blocker) {
 }
 
 void UiController::OnMessageCenterChanged() {
-  if (message_center_visible_ && message_center_->NotificationCount() == 0)
+  if (message_center_visible_ && message_center_->NotificationCount() == 0) {
     HideMessageCenterBubble();
+    return;
+  }
 
   if (popups_visible_ && !message_center_->HasPopupNotifications())
     HidePopupBubbleInternal();