arc: Fix race condition in applying modal state.

components/exo/shell_surface.h

 // Copyright 2015 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.
 
 #ifndef COMPONENTS_EXO_SHELL_SURFACE_H_
 #define COMPONENTS_EXO_SHELL_SURFACE_H_
 
 #include <deque>
 #include <memory>
 #include <string>
@@ skipping 329 matching lines @@
   aura::Window* shadow_underlay_ = nullptr;
   std::unique_ptr<ui::EventHandler> shadow_underlay_event_handler_;
   gfx::Rect shadow_content_bounds_;
   float shadow_background_opacity_ = 1.0;
   std::deque<Config> pending_configs_;
   std::unique_ptr<ash::WindowResizer> resizer_;
   std::unique_ptr<ScopedAnimationsDisabled> scoped_animations_disabled_;
   int top_inset_height_ = 0;
   int pending_top_inset_height_ = 0;
   bool shadow_underlay_in_surface_ = true;
+  bool system_modal_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(ShellSurface);
 };
 
 }  // namespace exo
 
 #endif  // COMPONENTS_EXO_SHELL_SURFACE_H_