[layoutng] Use the override width even when we're not a flex item

(And also use it for positioned boxes)

We want to use this for layoutng/legacy interop. For legacy layout,
there should be no change in behavior because flexbox is currently
the only caller of this function:
https://cs.chromium.org/search/?q=setOverrideLogicalContentWidth&sq=package:chromium&type=cs

Interestingly, the height version did not have this limitation already.

This patch also deleted an outdated FIXME comment.

R=atotic@chromium.org,eae@chromium.org
BUG=635619

Review-Url: https://codereview.chromium.org/2624143005
Cr-Commit-Position: refs/heads/master@{#443056}
1 file changed