Fix crash when removing all notifications by "Clear all" button.

ui/arc/notification/arc_custom_notification_view.cc

Index: ui/arc/notification/arc_custom_notification_view.cc
diff --git a/ui/arc/notification/arc_custom_notification_view.cc b/ui/arc/notification/arc_custom_notification_view.cc
index 805d31ccda71f532be2cdfc7f2d21d5fcf43772d..1730e917bf69a4256f0c3a74e7a9d6a9c08a923c 100644
--- a/ui/arc/notification/arc_custom_notification_view.cc
+++ b/ui/arc/notification/arc_custom_notification_view.cc
@@ -169,6 +169,8 @@ class ArcCustomNotificationView::ContentViewDelegate
   }
 
   bool IsPinned() const override {
+    if (!owner_->item_)
+      return false;
     return owner_->item_->pinned();
   }