[LayoutNG] Change NGPositionedFloat to have a NGBfcOffset.

... instead of a logical offset relative to its parent.

This simplifies the logic inside of ng_float_utils, and is convenient
for floats inside of lineboxes.

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I5b16df1f7a296a4222da7092a3d8290af2359e92
Reviewed-on: https://chromium-review.googlesource.com/719262
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508915}
8 files changed