mus: Allow SetModal to fail gracefully

components/mus/public/cpp/lib/in_flight_change.h

Index: components/mus/public/cpp/lib/in_flight_change.h
diff --git a/components/mus/public/cpp/lib/in_flight_change.h b/components/mus/public/cpp/lib/in_flight_change.h
index f848ab3b9fe4a2f1e780fc88b09454a248b1e9d5..13a634df8dbfb5846d28879c111da997927a0e29 100644
--- a/components/mus/public/cpp/lib/in_flight_change.h
+++ b/components/mus/public/cpp/lib/in_flight_change.h
@@ -253,6 +253,7 @@ class InFlightPredefinedCursorChange : public InFlightChange {
 class InFlightVisibleChange : public InFlightChange {
  public:
   InFlightVisibleChange(Window* window, const bool revert_value);
+  ~InFlightVisibleChange() override;
 
   // InFlightChange:
   void SetRevertValueFrom(const InFlightChange& change) override;
@@ -264,6 +265,19 @@ class InFlightVisibleChange : public InFlightChange {
   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_