[LayoutNG] Clean up #includes.

NGLayoutResult needs to know about NGUnpositionedFloat, and
NGUnpositionedFloat needs to know about NGLayoutResult, so some
forward-declaring is necessary, to avoid circular dependencies.

Also got a dependency on the rather heavy ComputedStyle out of a header
file.

Change-Id: I8f35bbb48bf39118abf093d8ffb6c8a44545ca57
Reviewed-on: https://chromium-review.googlesource.com/623651
Commit-Queue: Morten Stenshorne <mstensho@opera.com>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496008}
82 files changed