Converts WindowTreeClientTest to be in terms of aura

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 03c333e8f365cbe8eb568f28ca76010030177651..3808bf2cae80ff0574fe816fff56c3e730ff2da5 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -264,6 +264,11 @@ void WindowTreeClient::Embed(
     uint32_t flags,
     const ui::mojom::WindowTree::EmbedCallback& callback) {
   DCHECK(tree_);
+  if (!window->children().empty()) {

[msw, 2016/12/08 01:20:12]

nit: add a comment, explaining why only one child is allowed?

[sky, 2016/12/08 04:59:45]

You mean no children I believe. I added a comment.

+    callback.Run(false);
+    return;
+  }
+
   tree_->Embed(WindowMus::Get(window)->server_id(), std::move(client), flags,
                callback);
 }
@@ -595,9 +600,7 @@ void WindowTreeClient::OnWindowMusDestroyed(WindowMus* window, Origin origin) {
   for (auto change_id : in_flight_change_ids_to_remove)
     in_flight_map_.erase(change_id);
 
-  const bool was_root = roots_.erase(window) > 0;
-  if (!in_destructor_ && was_root && roots_.empty() && is_from_embed_)
-    delegate_->OnEmbedRootDestroyed(window->GetWindow());
+  roots_.erase(window);
 }
 
 void WindowTreeClient::OnWindowMusBoundsChanged(WindowMus* window,
@@ -1001,8 +1004,15 @@ void WindowTreeClient::OnWindowReordered(Id window_id,
 
 void WindowTreeClient::OnWindowDeleted(Id window_id) {
   WindowMus* window = GetWindowByServerId(window_id);
-  if (window)
+  if (!window)
+    return;
+
+  if (roots_.count(window)) {

[msw, 2016/12/08 01:20:12]

nit: add a comment explaining the two paths here

[sky, 2016/12/08 04:59:45]

Done.

+    window->PrepareForDestroy();
+    delegate_->OnEmbedRootDestroyed(GetWindowTreeHostMus(window));
+  } else {
     window->DestroyFromServer();
+  }
 }
 
 void WindowTreeClient::OnWindowVisibilityChanged(Id window_id, bool visible) {