Creates BackgroundShell and uses it in test

ui/views/mus/window_manager_connection.cc

Index: ui/views/mus/window_manager_connection.cc
diff --git a/ui/views/mus/window_manager_connection.cc b/ui/views/mus/window_manager_connection.cc
index 173572dde30ab3eca7e9cb4e11d466cb752326b1..9a0e5cfbdb75100b6ffe016e088ba109d3d487ce 100644
--- a/ui/views/mus/window_manager_connection.cc
+++ b/ui/views/mus/window_manager_connection.cc
@@ -43,6 +43,12 @@ WindowManagerConnection* WindowManagerConnection::Get() {
   return connection;
 }
 
+// static
+void WindowManagerConnection::Reset() {
+  delete Get();
+  lazy_tls_ptr.Pointer()->Set(nullptr);
+}
+
 mus::Window* WindowManagerConnection::NewWindow(
     const std::map<std::string, std::vector<uint8_t>>& properties) {
   return window_tree_connection_->NewTopLevelWindow(&properties);