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

ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h"
 
 #include <X11/Xatom.h>
 #include <X11/Xregion.h>
 #include <X11/Xutil.h>
 #include <X11/extensions/XInput2.h>
@@ skipping 1635 matching lines @@
     XChangeProperty(xdisplay_,
                     xwindow_,
                     atom_cache_.GetAtom("_NET_WM_USER_TIME"),
                     XA_CARDINAL,
                     32,
                     PropModeReplace,
                     reinterpret_cast<const unsigned char *>(&wm_user_time_ms),
                     1);
   }
 
+  x_map_window_was_called_ = true;
   XMapWindow(xdisplay_, xwindow_);
 
   // We now block until our window is mapped. Some X11 APIs will crash and
   // burn if passed |xwindow_| before the window is mapped, and XMapWindow is
   // asynchronous.
   if (ui::X11EventSource::GetInstance())
     ui::X11EventSource::GetInstance()->BlockUntilWindowMapped(xwindow_);
   window_mapped_ = true;
+  x_map_window_was_called_ = false;
 
   UpdateMinAndMaxSize();
 
   // Some WMs only respect maximize hints after the window has been mapped.
   // Check whether we need to re-do a maximization.
   if (should_maximize_after_map_) {
     Maximize();
     should_maximize_after_map_ = false;
   }
 }
@@ skipping 202 matching lines @@
         default:
           NOTREACHED();
       }
 
       // If we coalesced an event we need to free its cookie.
       if (num_coalesced > 0)
         XFreeEventData(xev->xgeneric.display, &last_event.xcookie);
       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_));
       break;
     }
     case UnmapNotify: {
       FOR_EACH_OBSERVER(DesktopWindowTreeHostObserverX11,
                         observer_list_,
                         OnWindowUnmapped(xwindow_));
       break;
@@ skipping 141 matching lines @@
   if (linux_ui) {
     ui::NativeTheme* native_theme = linux_ui->GetNativeTheme(window);
     if (native_theme)
       return native_theme;
   }
 
   return ui::NativeThemeAura::instance();
 }
 
 }  // namespace views