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Previously PatchElementHeader members were ordered by:
  [old_offset, new_offset, old_length, new_length].
This CL changes the order to
  [old_offset, old_length, new_offset, new_length]
to be consistent with PatchHeader's grouping by "old" and "new".

Also improve comments in patch_read_write_unittest.cc, and fix
input value in PatchElementTest.WrongExtraData.

Change-Id: I0dab56b7de1ba99f0a2f686dfebdf422a9d1c3b0
Reviewed-on: https://chromium-review.googlesource.com/1043073
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556076}