mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/test_utils.h

Index: components/mus/ws/test_utils.h
diff --git a/components/mus/ws/test_utils.h b/components/mus/ws/test_utils.h
index e10672bbd40d9a306c87b8093d1c3b220369f20c..1d4126157f2413444d64a600cc5c2ba1129a52ca 100644
--- a/components/mus/ws/test_utils.h
+++ b/components/mus/ws/test_utils.h
@@ -76,6 +76,9 @@ class WindowTreeTestApi {
   void ClearAck() { tree_->event_ack_id_ = 0; }
   void EnableCapture() { tree_->event_ack_id_ = 1u; }
 
+  void SetEventObserver(mojom::EventMatcherPtr matcher);
+  void ClearEventObserver();
+
  private:
   WindowTree* tree_;
 
@@ -290,6 +293,7 @@ class TestWindowTreeClient : public mus::mojom::WindowTreeClient {
   void OnWindowInputEvent(uint32_t event_id,
                           uint32_t window,
                           mojom::EventPtr event) override;
+  void OnEventObserved(mojom::EventPtr event) override;
   void OnWindowFocused(uint32_t focused_window_id) override;
   void OnWindowPredefinedCursorChanged(uint32_t window_id,
                                        mojom::Cursor cursor_id) override;