WTS: Code cleanup to improve readability.

* Update state.prevNodeUnmodified and state.currNodeUnmodified
  in the main serialization function and eliminate the confusion
  from updating it in SerializerState (but which only happens
  when the node generates an output that results in cb being
  invoked).

* Got rid of confusing trace output for current node (whose flag
  hasn't been updated yet).

* Emit trace output for diff-marker nodes as well.

* Emit trace output in serializeChildren as well.

* This doesn't affect any parser test results.

  But, doing this cleanup here minimizes diffs in the upcoming
  dom-diff and selser improvement patches.

Change-Id: I8e915f933721e1890a932afc91d3aec992b54f9e