[LayoutNG] Small cleanup + additional DCHECK from margin patch.

Just a small cleanup + additional DCHECK from when I looked at this code
again this morning.

There should be no functional changes.

Bug: 635619
Change-Id: Ib75cb30347f657a934d0151fa7d721e21316f5f1
Reviewed-on: https://chromium-review.googlesource.com/580191
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492338}
1 file changed