mus: Server-side implementation of modal windows

components/mus/ws/server_window.cc

Index: components/mus/ws/server_window.cc
diff --git a/components/mus/ws/server_window.cc b/components/mus/ws/server_window.cc
index 518c0640fe0abcbe126f08b9557f6b1ebbbb3ec6..b98108ef625943ec91e9903f785577131ef4082a 100644
--- a/components/mus/ws/server_window.cc
+++ b/components/mus/ws/server_window.cc
@@ -29,6 +29,7 @@ ServerWindow::ServerWindow(ServerWindowDelegate* delegate,
       parent_(nullptr),
       stacking_target_(nullptr),
       transient_parent_(nullptr),
+      is_modal_(false),
       visible_(false),
       cursor_id_(mojom::Cursor::CURSOR_NULL),
       opacity_(1),
@@ -241,6 +242,10 @@ void ServerWindow::RemoveTransientWindow(ServerWindow* child) {
                     OnTransientWindowRemoved(this, child));
 }
 
+void ServerWindow::SetAsModal() {
+  is_modal_ = true;
+}
+
 bool ServerWindow::Contains(const ServerWindow* window) const {
   for (const ServerWindow* parent = window; parent; parent = parent->parent_) {
     if (parent == this)