Property contentDocument (Frame/iFrame) is available in IE8 also

git-svn-id: svn://svn.code.sf.net/p/htmlunit/code/trunk@9120 5f5364db-9458-4db8-a492-e30667be6df6
force build

git-svn-id: svn://svn.code.sf.net/p/htmlunit/code/trunk@9121 5f5364db-9458-4db8-a492-e30667be6df6