[xml] Support Source Offset For Nodes

[Dan Lecocq <dan moz com>]

We recently needed support for retrieving the offset in the source document where nodes appear (particularly, anchors). I added it, mostly modeled after the source line number capturing, but I couldn't find any analogous tests. I'd be happy to add them if there's an example of similar testing, of course :-)

At any rate, here's the patch and hopefully it's helpful.

--

Dan Lecocq

Senior Software Engineer

w: moz.com

t: @danlecocq

Attachment: offset.patch
Description: Binary data