cros: Fix crash when adding a user after removing one.

Caused by a DCHECK crash from a pointer pointing to deallocated memory.

Bug: 866535
Change-Id: I96d76aeaeb8c89316cd6ffb237928cd2bfcfb744
Reviewed-on: https://chromium-review.googlesource.com/1165850Reviewed-by: Xiaoyin Hu <xiaoyinh@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581326}

ash/login/ui/lock_contents_view.cc

@@ -543,6 +543,7 @@ void LockContentsView::OnUsersChanged(
   // The debug view will potentially call this method many times. Make sure to
   // invalidate any child references.
   main_view_->RemoveAllChildViews(true /*delete_children*/);
+  primary_big_view_ = nullptr;
   opt_secondary_big_view_ = nullptr;
   users_list_ = nullptr;
   layout_actions_.clear();