No real changes, just avoid overloaded virtual wxWebView::SetPage().

Instead, have two public non-virtual SetPage() methods forwarding to a private
DoSetPage(), as usual.

This avoids the need for "using wxWebView::SetPage" which is needed to avoid
warnings about hiding the other base class virtual when implementing one of
them and which was forgotten in wxMSW version resulting in warnings when using
g++ to compile it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72200 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775