mus: Add debug key to print the server window hierarchy

components/mus/public/interfaces/window_manager.mojom

Index: components/mus/public/interfaces/window_manager.mojom
diff --git a/components/mus/public/interfaces/window_manager.mojom b/components/mus/public/interfaces/window_manager.mojom
index 558847161710bbc908488ed8deff08848cf5eb84..395fc98e0409117974128ac0ee79dff0da7f2efe 100644
--- a/components/mus/public/interfaces/window_manager.mojom
+++ b/components/mus/public/interfaces/window_manager.mojom
@@ -83,6 +83,7 @@ interface WindowManagerClient {
   // target window. The id is defined by the client and can be used to more
   // easily identify the accelerator's action. If an accelerator with the same
   // id or the same matcher already exists, then the accelerator is not added.
+  // Accelerator ids 1 << 31 and above are reserved for internal use.
   AddAccelerator(uint32 id, EventMatcher matcher) => (bool success);
   RemoveAccelerator(uint32 id);