Introduce NGFragmentWithOffset struct

This patch introduces |NGPhysicalFragmentWithOffset| as replacement of
|NGInlineFragmentIterator::Result| to use it for other places such as
|NGInlineFragmentTraversal::DescnednatsOf()|[1]


This patch is a a preparation of the patch[2].

[1] http://crrev.com/c/763188: [LayoutNG] Utilize DCHECK(CanUseInlineBox()) in
core/layout/
[2] http://crrev.com/c/795523: Adapt LayoutText::LinesBoundingBox() to LayoutNG

Bug: 591099
Change-Id: I6ec1f07e6316537a49b600dfa387291b5bc0969f
Reviewed-on: https://chromium-review.googlesource.com/799610
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520472}
4 files changed