Refators display related functionality into own class

components/mus/ws/window_tree.h

Index: components/mus/ws/window_tree.h
diff --git a/components/mus/ws/window_tree.h b/components/mus/ws/window_tree.h
index fed9da0f762bfabca4cc62e683aecd1dcc88eff0..5d2b73a9843751ef110512e170a47021694416a5 100644
--- a/components/mus/ws/window_tree.h
+++ b/components/mus/ws/window_tree.h
@@ -34,6 +34,7 @@ namespace ws {
 
 class AccessPolicy;
 class ConnectionManager;
+class DisplayManager;
 class Display;
 class ServerWindow;
 class TargetedEvent;
@@ -212,6 +213,9 @@ class WindowTree : public mojom::WindowTree,
     EMBED,
   };
 
+  DisplayManager* display_manager();
+  const DisplayManager* display_manager() const;
+
   // Used when this tree is the window manager.
   Display* GetDisplayForWindowManager();