event_utils_win_unittest.cc: add missing override

ui/events/win/event_utils_win_unittest.cc

Index: ui/events/win/event_utils_win_unittest.cc
diff --git a/ui/events/win/event_utils_win_unittest.cc b/ui/events/win/event_utils_win_unittest.cc
index f3cc7b98ac936e8f9897e5f1c1a40e74ea41e655..a4efce4fa5117119b48f0509e6dc63e346f94c0d 100644
--- a/ui/events/win/event_utils_win_unittest.cc
+++ b/ui/events/win/event_utils_win_unittest.cc
@@ -21,7 +21,7 @@ class TestWindow : public gfx::WindowImpl {
                             WPARAM w_param,
                             LPARAM l_param,
                             LRESULT& result,
-                            DWORD msg_map_id = 0) {
+                            DWORD msg_map_id = 0) override {
     return true;
   }