This ensures that the value returned from GetFieldRect() is always up to date,
even when this method is called from the user-defined wxEVT_SIZE handler, i.e.
before our own OnSize() could run.
Also remove the now unneeded hack with calling the base class OnSize() from
the statbar sample.
Closes#14268.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71365 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775