Allow parallel creation of windows

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index f6c28e6be901ab23bc1274c27272c6ab68f2b39c..8d860d5eb71bae3b199b0506634341ea3a6c9cec 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -880,11 +880,9 @@ void WindowTreeClient::OnEmbed(ClientSpecificId client_id,
     window_tree_host->InitHost();
     ConfigureWindowDataFromServer(window_tree_host, *root_data);
 
-    // TODO(tonikitoo): Fix the WindowTreeClientDelegate::OnEmbed API.
-    // In external window mode, this needs not to pass the ownership of the
-    // WindowTreeHostMus instance to the delegate_. A raw pointer should
-    // surface.
-    delegate_->OnEmbed(nullptr);
+    // Pass a raw pointer to WindowTreeHostMus, which helps delegate to identify
+    // which WindowTreeHostMus to use in case if there are many of them.
+    delegate_->OnEmbedRootReady(window_tree_host);
     return;
   }