Implement wxWindow::DoGetBorderSize() in wxUniv.

As some wxUniv classes implement DoGetBestClientSize(), the new code in
wxWindow::DoGetBestSize() implementation calls DoGetBorderSize() which asserts
because it's not implemented, making it impossible to even start the minimal
sample -- fix this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62002 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/univ/winuniv.cpp

@@ -728,6 +728,11 @@ wxSize wxWindow::DoGetBestClientSize() const
     return wxWindowNative::DoGetBestSize();
 }
 
+wxSize wxWindow::DoGetBorderSize() const
+{
+    return AdjustSize(wxSize(0, 0));
+}
+
 wxSize wxWindow::AdjustSize(const wxSize& size) const
 {
     wxSize sz = size;