Implement wxProgressDialog::DoGetSize() for the native dialog too

DoGetPosition() was done in 1ef1f8fda6,
implement DoGetSize() too now in order to give a chance Centre() (which
needs both position and size to work).

See #17768.

include/wx/msw/progdlg.h

@@ -44,6 +44,7 @@ public:
     virtual void SetIcons(const wxIconBundle& icons) wxOVERRIDE;
     virtual void DoMoveWindow(int x, int y, int width, int height) wxOVERRIDE;
     virtual void DoGetPosition(int *x, int *y) const wxOVERRIDE;
+    virtual void DoGetSize(int *width, int *height) const wxOVERRIDE;
     virtual void Fit() wxOVERRIDE;
 
     virtual bool Show( bool show = true ) wxOVERRIDE;