(Bug 67237): Fix citation numbering issue

* Process <ref> and <references> tag on the top-level DOM only
  and ignore the generateRefs pass when processing other content.

* This required a few fixes:
  - ensure that DOMPostProcessor knows about the top-level.
  - ensure that DOMVisitor knows about the top-level.
  - cleanup pass leaves behind the ref-marker metas from DOMs from
    non top-level content.
  - process nested references content.

* One of the references tests had incorrect parsed output. That test
  has been updated to reflect the correct output from this patch.

* Barack Obama seems to now have the correct numbering on references.

Change-Id: I5465721d2fc715f2168f267e773a446bc37d198b
Merge "(Bug 67237): Fix citation numbering issue"