[LayoutNG] NGLogicalDelta resolves the ambiguity of subtractions

Following the ambiguity discussion in r507157 (CL:701817):

"offset - offset" is ambiguous because both of below are true:
  offset + offset = offset
  offset + size = offset

NGLogicalDelta resolves this ambiguity by allowing implicit
conversions both to NGLogicalOffset and to NGLogicalSize.

Bug: 591099
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I725ba1cfdfc1a78a7754f01a96919589ceffb0bf
Reviewed-on: https://chromium-review.googlesource.com/706834
Commit-Queue: Koji Ishii <kojii@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@opera.com>
Cr-Commit-Position: refs/heads/master@{#507449}
4 files changed