b=934843 remove assertion that focus is never on popup windows r=roc

If an application has received focus from the window manager, but doesn't
reassign focus to a child window before closing its focus window, focus can
temporarily revert to PointerRoot before the window manager gives focus to
another window.  During PointerRoot focus, the window until the pointer has
focus even if it is an override-redirect window.

widget/gtk/nsWindow.cpp

@@ -2816,9 +2816,6 @@ nsWindow::OnButtonReleaseEvent(GdkEventButton *aEvent)
 void
 nsWindow::OnContainerFocusInEvent(GdkEventFocus *aEvent)
 {
-    NS_ASSERTION(mWindowType != eWindowType_popup,
-                 "Unexpected focus on a popup window");
-
     LOGFOCUS(("OnContainerFocusInEvent [%p]\n", (void *)this));
 
     // Unset the urgency hint, if possible