Serialize HTML + reparse HTML (dont reuse in-memory DOM) in wt2wt mode

* I wasted some time trying to come up with a reduced test case for
  a bug before realizing that the fostering was not taking place
  because parse.js reuses the parsed DOM (I'll investigate separately
  why fosterable content is being generated -- templated attribute
  metas or some such thing).

Change-Id: I5fb93dfcae513581cef3e2336d79ed64692c92d3