Extract the view of control buttons on notification into a separated class

ui/message_center/views/message_view.cc

Index: ui/message_center/views/message_view.cc
diff --git a/ui/message_center/views/message_view.cc b/ui/message_center/views/message_view.cc
index ed06b3ff7dd00f33bd4dbb15b205706c9ea496fb..340d9bcce8df1fe5269f693d111c8a21d052424f 100644
--- a/ui/message_center/views/message_view.cc
+++ b/ui/message_center/views/message_view.cc
@@ -235,6 +235,10 @@ void MessageView::OnCloseButtonPressed() {
   controller_->RemoveNotification(notification_id_, true);  // By user.
 }
 
+void MessageView::OnSettingsButtonPressed() {
+  controller_->ClickOnSettingsButton(notification_id_);
+}
+
 void MessageView::SetDrawBackgroundAsActive(bool active) {
   background_view_->background()->
       SetNativeControlColor(active ? kHoveredButtonBackgroundColor :