[LayoutNG] Introduce block child iterator.

This will be used for iterating of children while resuming layout from a
given break token.

See experimental patch for the multi-col approach: crrev/2693193002

BUG=635619

Review-Url: https://codereview.chromium.org/2706353004
Cr-Commit-Position: refs/heads/master@{#452333}
8 files changed