Add a missing "override".

No behavior change.

(Probably not really needed for the plugin because of crbug.com/484721
but nice to add anyways.)

BUG=467287
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/1128423003

Cr-Commit-Position: refs/heads/master@{#328842}

ui/views/win/hwnd_message_handler.h

@@ -77,7 +77,7 @@ const int WM_NCUAHDRAWFRAME = 0xAF;
                             WPARAM w_param, \
                             LPARAM l_param, \
                             LRESULT& l_result, \
-                            DWORD msg_map_id = 0) { \
+                            DWORD msg_map_id = 0) override { \
     base::WeakPtr<HWNDMessageHandler> ref(weak_factory.GetWeakPtr()); \
     BOOL old_msg_handled = msg_handled_; \
     BOOL ret = _ProcessWindowMessage(hwnd, msg, w_param, l_param, l_result, \