mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/window_manager_state.cc

Index: components/mus/ws/window_manager_state.cc
diff --git a/components/mus/ws/window_manager_state.cc b/components/mus/ws/window_manager_state.cc
index c81bb8cb86b78b4d1ccfc2e6e0649a9e30cc96f3..3f0dc21a2817abdb1cc3372d33c8810438ee9cae 100644
--- a/components/mus/ws/window_manager_state.cc
+++ b/components/mus/ws/window_manager_state.cc
@@ -329,9 +329,16 @@ void WindowManagerState::DispatchInputEventToWindowImpl(
     event_awaiting_input_ack_ = ui::Event::Clone(event);
     post_target_accelerator_ = accelerator;
   }
+
+  // Ignore |tree| because it will receive the event via normal dispatch.
+  window_server()->SendToEventObservers(event, user_id_, tree);
+
   tree->DispatchInputEvent(target, event);
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// EventDispatcherDelegate:
+
 void WindowManagerState::OnAccelerator(uint32_t accelerator_id,
                                        const ui::Event& event) {
   DCHECK(IsActive());
@@ -383,5 +390,10 @@ void WindowManagerState::DispatchInputEventToWindow(ServerWindow* target,
                                  weak_accelerator);
 }
 
+void WindowManagerState::OnEventTargetNotFound(const ui::Event& event) {
+  window_server()->SendToEventObservers(event, user_id_,
+                                        nullptr /* ignore_tree */);
+}
+
 }  // namespace ws
 }  // namespace mus