[LayoutNG] Add fields required for new multi-col approach.

Adds:
1) UsedBlockSize() - used to communicate how much block size was used by
  preceeding fragments.

2) ChildBreakTokens() - used for resuming children in fragmented flow.

3) IsFinished() - if true the node cannot produce any more fragments. This
  shouldn't have any child break tokens.

BUG=635619

Review-Url: https://codereview.chromium.org/2697843004
Cr-Commit-Position: refs/heads/master@{#451233}
2 files changed