Makes DesktopWindowTreeHostMus call OnNativeWidgetCreated()

ui/views/mus/desktop_window_tree_host_mus.cc

Index: ui/views/mus/desktop_window_tree_host_mus.cc
diff --git a/ui/views/mus/desktop_window_tree_host_mus.cc b/ui/views/mus/desktop_window_tree_host_mus.cc
index 29a31d919c34ac28aa88204e517a4e79d1603dd2..478dfb93fa70c09ad16107f3db46aa0226626030 100644
--- a/ui/views/mus/desktop_window_tree_host_mus.cc
+++ b/ui/views/mus/desktop_window_tree_host_mus.cc
@@ -45,7 +45,9 @@ void DesktopWindowTreeHostMus::Init(aura::Window* content_window,
                                     const Widget::InitParams& params) {}
 
 void DesktopWindowTreeHostMus::OnNativeWidgetCreated(
-    const Widget::InitParams& params) {}
+    const Widget::InitParams& params) {
+  native_widget_delegate_->OnNativeWidgetCreated(true);
+}
 
 std::unique_ptr<corewm::Tooltip> DesktopWindowTreeHostMus::CreateTooltip() {
   return base::MakeUnique<corewm::TooltipAura>();