Add a crash for debugging when we get into a bad state.

ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
index 2b0f75652ed29a0efafe6783f47230401f2bbfc8..7889e4767e34e499cec8cd6f7261607df108e904 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc
@@ -1653,6 +1653,7 @@ void DesktopWindowTreeHostX11::MapWindow(ui::WindowShowState show_state) {
                     1);
   }
 
+  x_map_window_was_called_ = true;
   XMapWindow(xdisplay_, xwindow_);
 
   // We now block until our window is mapped. Some X11 APIs will crash and
@@ -1661,6 +1662,7 @@ void DesktopWindowTreeHostX11::MapWindow(ui::WindowShowState show_state) {
   if (ui::X11EventSource::GetInstance())
     ui::X11EventSource::GetInstance()->BlockUntilWindowMapped(xwindow_);
   window_mapped_ = true;
+  x_map_window_was_called_ = false;
 
   UpdateMinAndMaxSize();
 
@@ -1883,6 +1885,10 @@ uint32_t DesktopWindowTreeHostX11::DispatchEvent(
       break;
     }
     case MapNotify: {
+      CHECK(x_map_window_was_called_)
+          << "Received MapNotify event despite never calling XMapWindow(). "
+          << "(This is debugging state for crbug.com/381732.)";
+
       FOR_EACH_OBSERVER(DesktopWindowTreeHostObserverX11,
                         observer_list_,
                         OnWindowMapped(xwindow_));