Allow parallel creation of windows

services/ui/demo/window_tree_data.cc

Index: services/ui/demo/window_tree_data.cc
diff --git a/services/ui/demo/window_tree_data.cc b/services/ui/demo/window_tree_data.cc
index 65055a8a6dfe65f1b10cba3f2f625bbb9d0fcde5..4f5fb2fc95c5cdbe7b8b4246f04f99696df8d505 100644
--- a/services/ui/demo/window_tree_data.cc
+++ b/services/ui/demo/window_tree_data.cc
@@ -68,6 +68,7 @@ aura::Window* WindowTreeData::bitmap_window() {
 
 void WindowTreeData::Init(
     std::unique_ptr<aura::WindowTreeHostMus> window_tree_host) {

[fwang, 2017/03/16 08:59:49]

So I think you can remove the parameter and assume DCHECK(window_tree_host_)
here.

Then WindowTreeData could have a constructor that accepts a
std::unique_ptr<aura::WindowTreeHostMus> parameter and performs what is in the
if (window_tree_host) { } below.

Finally, MusDemoInternal will be able to use the constructor to append the
WindowTreeData and then call Init on the data without passing any
std::unique_ptr<aura::WindowTreeHostMus>. And MusDemoExternal won't need to pass
a nullptr (which is a bit weird).

[msisov(use msisov igalia.com), 2017/03/16 09:21:32]

If window_tree_host->InitHost(); and window_tree_host->Show() go 
into ctor, then in case of external demo, those two would be called before
OnEmbedRootReady, which is wrong, I guess.

+  DCHECK(!IsInitialized());
   if (window_tree_host) {
     window_tree_host->InitHost();
     window_tree_host->Show();