mus: Allow SetModal to fail gracefully

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

Index: components/mus/public/cpp/lib/in_flight_change.cc
diff --git a/components/mus/public/cpp/lib/in_flight_change.cc b/components/mus/public/cpp/lib/in_flight_change.cc
index 6b01224047340c015b5c8fda69fec3285802d309..2c9a1c9b7eadd843e5cc75e3a2c8a1226f1ecd5e 100644
--- a/components/mus/public/cpp/lib/in_flight_change.cc
+++ b/components/mus/public/cpp/lib/in_flight_change.cc
@@ -186,4 +186,15 @@ void InFlightVisibleChange::Revert() {
   WindowPrivate(window()).LocalSetVisible(revert_visible_);
 }
 
+// InFlightSetModalChange ------------------------------------------------------
+
+InFlightSetModalChange::InFlightSetModalChange(Window* window)
+    : InFlightChange(window, ChangeType::SET_MODAL) {}
+
+void InFlightSetModalChange::SetRevertValueFrom(const InFlightChange& change) {}
+
+void InFlightSetModalChange::Revert() {
+  WindowPrivate(window()).LocalUnsetModal();
+}
+
 }  // namespace mus