Implement a MusBrowserFrame.

ui/views/mus/native_widget_mus.h

Index: ui/views/mus/native_widget_mus.h
diff --git a/ui/views/mus/native_widget_mus.h b/ui/views/mus/native_widget_mus.h
index bedbba578dc135124715bf07a0e1f9e9af4ff0ec..504d4c50d86d6985121dd904ec0a5e53256816b3 100644
--- a/ui/views/mus/native_widget_mus.h
+++ b/ui/views/mus/native_widget_mus.h
@@ -17,6 +17,7 @@
 namespace aura {
 namespace client {
 class DefaultCaptureClient;
+class WindowTreeClient;
 }
 class Window;
 }
@@ -199,6 +200,7 @@ class NativeWidgetMus : public internal::NativeWidgetPrivate,
   aura::Window* content_;
   scoped_ptr<wm::FocusController> focus_client_;
   scoped_ptr<aura::client::DefaultCaptureClient> capture_client_;
+  scoped_ptr<aura::client::WindowTreeClient> window_tree_client_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeWidgetMus);
 };