Simplify and make more robust wxHtmlWindow layout logic

Rewrite CreateLayout() using SetVirtualSize() to avoid trying to detect
whether the vertical scrollbar is shown manually. This is simpler and
avoids the problem of entering infinite loop if the scrollbar size used
by this function (based on wxSystemSettings::GetMetric(wxSYS_VSCROLL_X))
wasn't correct, as used to be the case for wxGTK until the recent commit
900752b152 and might still be the case in
the other ports.

Closes #18141.

docs/changes.txt

@@ -95,6 +95,7 @@ All (GUI):
 - Revert to left-aligning wxSpinCtrl contents by default.
 - Make wxRibbonButtonBar buttons more customizable (Max Maisel).
 - Add wxHtmlEasyPrinting::SetPromptMode() (pavel-t).
+- Fix possible infinite loop in wxHtmlWindow layout (trivia21).
 
 wxGTK: