Allow parallel creation of windows

ui/aura/mus/window_tree_client.cc

 // Copyright 2014 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.
 
 #include "ui/aura/mus/window_tree_client.h"
 
 #include <stddef.h>
 
 #include <string>
 #include <utility>
@@ skipping 862 matching lines @@
     DCHECK(tree_ptr_);
     DCHECK(!tree);
 
     auto it = windows_.find(focused_window_id);
     DCHECK(it != windows_.end());
 
     WindowTreeHostMus* window_tree_host = GetWindowTreeHostMus(it->second);
     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;
   }
 
   DCHECK(!tree_ptr_);
   DCHECK(tree);
   tree_ptr_ = std::move(tree);
 
   is_from_embed_ = true;
 
   if (window_manager_delegate_) {
@@ skipping 983 matching lines @@
   return ScheduleInFlightChange(base::MakeUnique<InFlightCaptureChange>(
       this, capture_synchronizer_.get(), window));
 }
 
 uint32_t WindowTreeClient::CreateChangeIdForFocus(WindowMus* window) {
   return ScheduleInFlightChange(base::MakeUnique<InFlightFocusChange>(
       this, focus_synchronizer_.get(), window));
 }
 
 }  // namespace aura