Implement a MusBrowserFrame.

ui/views/mus/window_manager_connection.h

Index: ui/views/mus/window_manager_connection.h
diff --git a/ui/views/mus/window_manager_connection.h b/ui/views/mus/window_manager_connection.h
index 38a90d08079bd185d764a7d68941511f2676cf5c..497bbb96a33829c44283aea59aa2be17c1852721 100644
--- a/ui/views/mus/window_manager_connection.h
+++ b/ui/views/mus/window_manager_connection.h
@@ -34,6 +34,8 @@ class WindowManagerConnection : public mus::WindowTreeDelegate {
                      mojo::ApplicationImpl* app);
   static WindowManagerConnection* Get();
 
+  mojo::ApplicationImpl* app() { return app_; }
+
   mus::Window* NewWindow(const std::map<std::string,
                          std::vector<uint8_t>>& properties);