Fixes to findBuilderModifiedTags to correct DSR errors.

* '''''x''''' generates a misnested token stream of i and b tags
  and it is hard to do this correctly in the quote handler without
  building the DOM.  The <i>, <b>, x, </i>, </b> is then fixed up
  by the tree builder into <i><b>x</b></i> which is effectively
  an auto-insert of the </b> inside <i> and a delete of the </b>
  tag after <i>.
* Fixed up the 'findBuilderModifiedTags' to recognize this scenario
  correctly and mark the b-tag with autoInsertedEnd and add a
  placeholder tag for the deleted b.
* This change fixes DSR computation for all the parser tests where
  s/cs mismatches were being reported and also greens one more
  wt2wt test.
* Once we have serializer refactoring regressions fixed to our
  satisfaction, we will turn off serialization of the placeholder
  tags from auto-deleted tags since this is error-prone in the
  face of editor modifications.  To be done later.

Change-Id: Ia5c7be1d4669199cea2a25916fd0c584aafecd2b
Merge "Fixes to findBuilderModifiedTags to correct DSR errors."