Fix long-standing serialization diffing bug in roundtrip-test.js

* doubleRoundTripDiff was unconditionally using outerHTML on nodes
  to compute a html string and compare them.

* This fails when the nodes in a DSR range contains text and
  comment nodes and causes artificial semantic diffs to be
  reported.

* Test case: node roundtrip-test.js --prefix de Nahe

Change-Id: Idbf53b84ffe59ad40ad3b3ec844632b9beed1265
Merge "Fix long-standing serialization diffing bug in roundtrip-test.js"