mus: Allow SetModal to fail gracefully

components/mus/public/cpp/lib/in_flight_change.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_MUS_PUBLIC_CPP_LIB_IN_FLIGHT_CHANGE_H_
 #define COMPONENTS_MUS_PUBLIC_CPP_LIB_IN_FLIGHT_CHANGE_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 235 matching lines @@
 
  private:
   mojom::Cursor revert_cursor_;
 
   DISALLOW_COPY_AND_ASSIGN(InFlightPredefinedCursorChange);
 };
 
 class InFlightVisibleChange : public InFlightChange {
  public:
   InFlightVisibleChange(Window* window, const bool revert_value);
+  ~InFlightVisibleChange() override;
 
   // InFlightChange:
   void SetRevertValueFrom(const InFlightChange& change) override;
   void Revert() override;
 
  private:
   bool revert_visible_;
 
   DISALLOW_COPY_AND_ASSIGN(InFlightVisibleChange);
 };
 
+class InFlightSetModalChange : public InFlightChange {
+ public:
+  explicit InFlightSetModalChange(Window* window);
+  ~InFlightSetModalChange() override;
+
+  // InFlightChange:
+  void SetRevertValueFrom(const InFlightChange& change) override;
+  void Revert() override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(InFlightSetModalChange);
+};
+
 }  // namespace mus
 
 #endif  // COMPONENTS_MUS_PUBLIC_CPP_LIB_IN_FLIGHT_CHANGE_H_