Allow parallel creation of windows

services/ui/demo/mus_demo.cc

Index: services/ui/demo/mus_demo.cc
diff --git a/services/ui/demo/mus_demo.cc b/services/ui/demo/mus_demo.cc
index 7afba5693af4cae100e7e13c0f09af916494b989..0100f3f1a2d34c72af60fc87e62b78d3e6fc82bd 100644
--- a/services/ui/demo/mus_demo.cc
+++ b/services/ui/demo/mus_demo.cc
@@ -38,7 +38,6 @@ bool MusDemo::HasPendingWindowTreeData() const {
 
 void MusDemo::AppendWindowTreeData(
     std::unique_ptr<WindowTreeData> window_tree_data) {
-  DCHECK(!HasPendingWindowTreeData());
   window_tree_data_list_.push_back(std::move(window_tree_data));
 }
 
@@ -48,15 +47,28 @@ void MusDemo::InitWindowTreeData(
   window_tree_data_list_.back()->Init(std::move(window_tree_host));
 }
 
+void MusDemo::InitWindowTreeDataExternal(
+    aura::WindowTreeHostMus* window_tree_host) {
+  DCHECK(HasPendingWindowTreeData());
+  auto window_tree_data = FindWindowTreeData(window_tree_host);
+  (*window_tree_data)->Init(nullptr);
+}
+
 void MusDemo::RemoveWindowTreeData(aura::WindowTreeHostMus* window_tree_host) {
   DCHECK(window_tree_host);
+  auto window_tree_data = FindWindowTreeData(window_tree_host);
+  window_tree_data_list_.erase(window_tree_data);
+}
+
+std::vector<std::unique_ptr<WindowTreeData>>::iterator
+MusDemo::FindWindowTreeData(aura::WindowTreeHostMus* window_tree_host) {
   auto it =
       std::find_if(window_tree_data_list_.begin(), window_tree_data_list_.end(),
                    [window_tree_host](std::unique_ptr<WindowTreeData>& data) {
                      return data->WindowTreeHost() == window_tree_host;
                    });
   DCHECK(it != window_tree_data_list_.end());
-  window_tree_data_list_.erase(it);
+  return it;
 }
 
 void MusDemo::OnStart() {