Change the telemetry_chrome_hangs pig to also collect late writes.

Things to note:
* My first pig file. It is probably "wrong".
* This selects only new format pig files.
* It outputs the result as a pig map, so the python script also has to be
  updated. Sending this first so that we can do the review in parallel.
* It switches to a white list of fields to massively reduce the size of the
  data that is output. Right now the only additional data is the addon list.
* If a ping has both a hang and a late write, it gets split. I think this is
  fine. I is hard to see what extra benefit we would get from knowing that a
  single ping had a hang and a late write.