mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/window_server.h

Index: components/mus/ws/window_server.h
diff --git a/components/mus/ws/window_server.h b/components/mus/ws/window_server.h
index e8318093ea41f591c33a08dedd4d013f6b6c9914..13fbfde26c8c5d8dcf319d78225e2c90f41b06c0 100644
--- a/components/mus/ws/window_server.h
+++ b/components/mus/ws/window_server.h
@@ -186,6 +186,9 @@ class WindowServer : public ServerWindowDelegate,
   void ProcessWillChangeWindowPredefinedCursor(ServerWindow* window,
                                                int32_t cursor_id);
 
+  // Sends an event to all WindowTrees that are observing events.
+  void SendToEventObservers(const ui::Event& event);
+
  private:
   friend class Operation;