mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/window_tree_client_unittest.cc

Index: components/mus/ws/window_tree_client_unittest.cc
diff --git a/components/mus/ws/window_tree_client_unittest.cc b/components/mus/ws/window_tree_client_unittest.cc
index 0ae95d650994698f605214e2371129b51fab8910..e09fa46c5e617d545bcdfc9990282d3e43b740f6 100644
--- a/components/mus/ws/window_tree_client_unittest.cc
+++ b/components/mus/ws/window_tree_client_unittest.cc
@@ -365,6 +365,7 @@ class TestWindowTreeClientImpl : public mojom::WindowTreeClient,
     // Don't log input events as none of the tests care about them and they
     // may come in at random points.
   }
+  void OnEventObserved(EventPtr event) override {}
   void OnWindowSharedPropertyChanged(uint32_t window,
                                      const String& name,
                                      Array<uint8_t> new_data) override {