[LayoutNG] Change anonymous LayoutBlockFlow to LayoutNGBlockFlow

Additionally this resolves an incoming margin strut once we know the inline node
isn't empty. In this respect we treat it like a zero-height div.

This regresses some tests that were using the existing LayoutBlockFlow layout.
I.e. when we are inside a LayoutTableCell.

BUG=635619

Review-Url: https://codereview.chromium.org/2851983002
Cr-Commit-Position: refs/heads/master@{#468470}
4 files changed