Make wxOwnerDrawnComboBox::DoGetBestSize() twice as fast.

Don't call the very expensive wxComboCtrlBase::DoGetBestSize() method
when not needed - it calls wxComboCtrlBase::DoGetSizeFromTextSize(),
which is very expensive.

Also fix the code so that it doesn't apply GetSizeFromTextSize() twice
(once in the base class' DoGetBestSize(), once here).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74644 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775