Add a basic browser to mash that uses the navigation service.

ui/views/mus/native_widget_mus.cc

Index: ui/views/mus/native_widget_mus.cc
diff --git a/ui/views/mus/native_widget_mus.cc b/ui/views/mus/native_widget_mus.cc
index 230849596946b0ef8d67ab30e4eabc167f88c98b..352e7ea503040bb893c49b3d93c1bd40cb6c70b0 100644
--- a/ui/views/mus/native_widget_mus.cc
+++ b/ui/views/mus/native_widget_mus.cc
@@ -472,11 +472,11 @@ void NativeWidgetMus::OnPlatformWindowClosed() {
   window_tree_client_.reset();  // Uses |content_|.
   capture_client_.reset();      // Uses |content_|.
 
-  cursor_manager_.reset();      // Uses |window_|.
-
   window_tree_host_->RemoveObserver(this);
   window_tree_host_.reset();
 
+  cursor_manager_.reset();  // Uses |window_|.
+
   mus_window_observer_.reset(nullptr);
 
   window_ = nullptr;