mus: Allow SetModal to fail gracefully

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

 // 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.
 
 #include "components/mus/public/cpp/lib/in_flight_change.h"
 
 #include "components/mus/public/cpp/lib/window_private.h"
 #include "components/mus/public/cpp/lib/window_tree_client_impl.h"
 #include "components/mus/public/cpp/window_tree_connection.h"
 
@@ skipping 168 matching lines @@
 
 void InFlightVisibleChange::SetRevertValueFrom(const InFlightChange& change) {
   revert_visible_ =
       static_cast<const InFlightVisibleChange&>(change).revert_visible_;
 }
 
 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