mus: Add EventObserver to allow passively listening to UI events

components/mus/public/cpp/tests/test_window_tree.h

Index: components/mus/public/cpp/tests/test_window_tree.h
diff --git a/components/mus/public/cpp/tests/test_window_tree.h b/components/mus/public/cpp/tests/test_window_tree.h
index 2910dc6529c431bbd9adf6bd380d19291fca6361..b3f08cd5c79081117daf2016def43c1a1ef876de 100644
--- a/components/mus/public/cpp/tests/test_window_tree.h
+++ b/components/mus/public/cpp/tests/test_window_tree.h
@@ -77,6 +77,8 @@ class TestWindowTree : public mojom::WindowTree {
                      const GetWindowTreeCallback& callback) override;
   void SetCapture(uint32_t change_id, uint32_t window_id) override;
   void ReleaseCapture(uint32_t change_id, uint32_t window_id) override;
+  void SetEventObserver(mojom::EventMatcherPtr matcher,
+                        uint32_t observer_id) override;
   void Embed(uint32_t window_id,
              mojom::WindowTreeClientPtr client,
              const EmbedCallback& callback) override;