Fixes bug in handling restacking because of transients

ui/aura/mus/window_mus.h

 // Copyright 2016 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 UI_AURA_MUS_WINDOW_MUS_H_
 #define UI_AURA_MUS_WINDOW_MUS_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 82 matching lines @@
   // Called in the rare case when WindowTreeClient needs to change state and
   // can't go through one of the SetFooFromServer() functions above. Generally
   // because it needs to call another function that as a side effect changes the
   // window. Once the call to the underlying window has completed the returned
   // object should be destroyed.
   virtual std::unique_ptr<WindowMusChangeData> PrepareForServerBoundsChange(
       const gfx::Rect& bounds) = 0;
   virtual std::unique_ptr<WindowMusChangeData> PrepareForServerVisibilityChange(
       bool value) = 0;
 
+  // See TransientWindowClientObserver::OnWillRestackTransientChildAbove() for
+  // details on this and OnTransientRestackDone().
+  virtual void PrepareForTransientRestack(WindowMus* window) = 0;
+  virtual void OnTransientRestackDone(WindowMus* window) = 0;
+
   virtual void NotifyEmbeddedAppDisconnected() = 0;
 
  private:
   // Just for set_server_id(), which other places should not call.
   friend class WindowTreeClient;
 
   void set_server_id(Id id) { server_id_ = id; }
 
   Id server_id_ = kInvalidServerId;
   const WindowMusType window_mus_type_;
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_WINDOW_MUS_H_