mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/window_server.cc

Index: components/mus/ws/window_server.cc
diff --git a/components/mus/ws/window_server.cc b/components/mus/ws/window_server.cc
index 6d73462cafb6f96f7ac2699338ae23c3cb51f02b..cb6b0f7de6accf168a0545076ccfd0861fc06c82 100644
--- a/components/mus/ws/window_server.cc
+++ b/components/mus/ws/window_server.cc
@@ -399,6 +399,16 @@ void WindowServer::ProcessWillChangeWindowPredefinedCursor(ServerWindow* window,
     display->OnCursorUpdated(window);
 }
 
+void WindowServer::SendToEventObservers(const ui::Event& event,
+                                        const UserId& user_id,
+                                        WindowTree* ignore_tree) {
+  for (auto& pair : tree_map_) {
+    WindowTree* tree = pair.second.get();
+    if (tree->user_id() == user_id && tree != ignore_tree)
+      tree->SendToEventObserver(event);
+  }
+}
+
 void WindowServer::ProcessViewportMetricsChanged(
     Display* display,
     const mojom::ViewportMetrics& old_metrics,