[LayoutNG] Introduce NGPositionedFloat.

This pulls out the positioned fragment info from NGFloatingObject.
This allows for in a followup to perform layout on a NGUnpositionedFloat
post positioning (to allow for fragmentation).

BUG=635619

Review-Url: https://codereview.chromium.org/2872593003
Cr-Commit-Position: refs/heads/master@{#470194}
13 files changed