don't use setBaseAndExtent anymore. webkit has supported addRange for years; jparent says so https://trac.webkit.org/changeset/18332

Hello @nicks, 

Please review the following commits I made in branch 'nick-baseandextent'.

36c7ccc633e9b98319a2f6c01f6c73cccfbdd179 (2014-05-20 17:27:36 -0400)
don't use setBaseAndExtent anymore. webkit has supported addRange for years; jparent says so https://trac.webkit.org/changeset/18332

R=@nicks