WTS Separators: Simplify indent-pre suppressing checks

* Eliminated the notion of LeadingWSAcceptingTags by simplifying
  the test that checks if a node suppresses indent-pre for any
  preceding whitespace.

* Also, fixed one more place where diff-marker snuck in and
  blocked traversal of DOM nodes. This fix seems to have
  improved output of one selser test and eliminated
  one selser failure.

Change-Id: I13c439552f5cf4aefa2e8f0bd293f50876566779