Fix newline constraints for mw:Placeholder/* metas

* mw:Placeholder meta nodes need not be serialized onto their own
  line even if they are newly added elements.

* After data-parsoid was stripped in 320caa30, these kind of metas
  in templated content was being treated as newly inserted content
  and would get serialized onto new lines.

  Try "node roundtrip-test.js --prefix kowiki 민아" which is run
  in roundtrip-test mode (no edits). In this mode, a
  mw:Placeholder/StrippedTag meta without a data-parsoid attribute
  was being serialized on a new line.

* This patch fixes the newline constraints and ensures that
  the above page now roundtrips perfectly.

Change-Id: I1daac63f797b2ede2bcb4903e53ee43034b00f99
Merge "Fix newline constraints for mw:Placeholder/* metas"