[LayoutNG] Remove NGFragment::Offset in favour of ConvertToLogical

This was something I found yesterday, while looking at the remaining
ng-bot failures for the line-by-line patch.

We weren't calculating the NGFragment::Offset correctly (impossible
without a parent size), so I removed it in favour of:
NGPhysicalOffset::ConvertToLogical.

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I88196b26d6702acf4c3b65a386eba04e2d4d1d33
Reviewed-on: https://chromium-review.googlesource.com/726299
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509866}
7 files changed