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..bcf891814d0eeaf676a887014fa4fc9cad0ec169 100644
--- a/components/mus/ws/window_server.h
+++ b/components/mus/ws/window_server.h
@@ -186,6 +186,12 @@ class WindowServer : public ServerWindowDelegate,
   void ProcessWillChangeWindowPredefinedCursor(ServerWindow* window,
                                                int32_t cursor_id);
 
+  // Sends an |event| to all WindowTrees belonging to |user_id| that might be
+  // observing events. Skips |ignore_tree| if it is non-null.
+  void SendToEventObservers(const ui::Event& event,
+                            const UserId& user_id,
+                            WindowTree* ignore_tree);
+
  private:
   friend class Operation;