mus: Server-side implementation of modal windows

components/mus/ws/window_tree.h

Index: components/mus/ws/window_tree.h
diff --git a/components/mus/ws/window_tree.h b/components/mus/ws/window_tree.h
index 378186de1abc9059b36510362b7df27570c2409b..509643ef1490c8183d24a07dcb61311568d097fd 100644
--- a/components/mus/ws/window_tree.h
+++ b/components/mus/ws/window_tree.h
@@ -122,6 +122,7 @@ class WindowTree : public mojom::WindowTree,
                  const ClientWindowId& child_id);
   bool AddTransientWindow(const ClientWindowId& window_id,
                           const ClientWindowId& transient_window_id);
+  bool SetAsModal(const ClientWindowId& window_id);
   std::vector<const ServerWindow*> GetWindowTree(
       const ClientWindowId& window_id) const;
   bool SetWindowVisibility(const ClientWindowId& window_id, bool visible);
@@ -306,6 +307,7 @@ class WindowTree : public mojom::WindowTree,
                           Id transient_window) override;
   void RemoveTransientWindowFromParent(uint32_t change_id,
                                        Id transient_window_id) override;
+  void SetAsModal(uint32_t change_id, Id window_id) override;
   void ReorderWindow(uint32_t change_Id,
                      Id window_id,
                      Id relative_window_id,