Fix erasing wxHtmlWindow background in wxUniv.

Fix erasing wxHtmlWindow background in wxUniv.

Prevent the default wxWindow-level wxEVT_ERASE_BACKGROUND handler from being
used in wxUniv for wxHtmlWindow. This is unnecessary as it has its own handler
anyhow and also doesn't work for some reason as erasing wxMemoryDC by drawing
a solid rectangle over it seems to be broken in at least wxX11.

Work around this problem by erasing the background in wxHtmlWindow itself if
no user-defined (as opposed to any, including one defined in wxWindow itself)
handler for this event exists.

Closes #13880.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71300 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775