Update wxHtmlWindow on DPI change

This is not ideal, as resetting the page contents loses the current
selection and redoes a lot of work that could be avoided, but it's
(much) better than nothing, as it fixes the window appearance after e.g.
moving it to another monitor.

Closes #18564.

include/wx/html/htmlwin.h

@@ -405,6 +405,7 @@ protected:
     void OnPaint(wxPaintEvent& event);
     void OnEraseBackground(wxEraseEvent& event);
     void OnSize(wxSizeEvent& event);
+    void OnDPIChanged(wxDPIChangedEvent& event);
     void OnMouseMove(wxMouseEvent& event);
     void OnMouseDown(wxMouseEvent& event);
     void OnMouseUp(wxMouseEvent& event);