ui: Cleanup class/struct forward declarations

ash/mus/window_manager.cc

Index: ash/mus/window_manager.cc
diff --git a/ash/mus/window_manager.cc b/ash/mus/window_manager.cc
index f02e2c2ed427f6ed0482cf31c69d179fe953e9ee..e2c97177788e6d90670efabff62a24cc66a8054b 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -60,6 +60,7 @@ void WindowManager::Init(
     connector_->ConnectToInterface("service:ui", &display_controller_);
 
   screen_ = base::MakeUnique<display::ScreenBase>();
+  display::Screen::SetScreenInstance(screen_.get());
 
   pointer_watcher_event_router_.reset(
       new views::PointerWatcherEventRouter(window_tree_client_.get()));
@@ -156,10 +157,10 @@ RootWindowController* WindowManager::CreateRootWindowController(
   // notify DisplayObservers. Classic ash does this by making sure
   // WindowTreeHostManager is added as a DisplayObserver early on.
   std::unique_ptr<display::DisplayListObserverLock> display_lock =
-      screen_->display_list()->SuspendObserverUpdates();
-  const bool is_first_display = screen_->display_list()->displays().empty();
+      screen_->display_list().SuspendObserverUpdates();
+  const bool is_first_display = screen_->display_list().displays().empty();
   // TODO(sky): should be passed whether display is primary.
-  screen_->display_list()->AddDisplay(
+  screen_->display_list().AddDisplay(
       display, is_first_display ? display::DisplayList::Type::PRIMARY
                                 : display::DisplayList::Type::NOT_PRIMARY);
 
@@ -175,7 +176,7 @@ RootWindowController* WindowManager::CreateRootWindowController(
   for (auto& observer : observers_)
     observer.OnRootWindowControllerAdded(root_window_controller);
 
-  for (auto& observer : *screen_->display_list()->observers())
+  for (auto& observer : *screen_->display_list().observers())
     observer.OnDisplayAdded(root_window_controller->display());
 
   return root_window_controller;
@@ -236,6 +237,9 @@ void WindowManager::Shutdown() {
 
   window_tree_client_.reset();
   window_manager_client_ = nullptr;
+
+  DCHECK_EQ(screen_.get(), display::Screen::GetScreen());
+  display::Screen::SetScreenInstance(nullptr);
 }
 
 WindowManager::RootWindowControllers::iterator