[LayoutNG]  Add out-of-flow descendants and offsets to NGPhysicalFragment

More prep work for abspos:
  Adds a list of out-of-flow descendants and their offsets to
  NGPhysicalFragment.

NGBlockLayoutAlgorithm needs to pass boxes it was unable to place
(out-of-flow) to its parent.
This will be done by storing unplaced boxes on the generated fragment.

BUG=635619

Review-Url: https://codereview.chromium.org/2521833002
Cr-Commit-Position: refs/heads/master@{#433945}
6 files changed