[LayoutNG] Part 1 of 3: Move block_size to NGContainerFragmentBuilder

Also removes obsolete comment

All 3 parts of CL will do the following:

Inline blocks can be OOFDescendants, and can contain OOFDescendants.
These changes move NGFragment OOF methods out of NGFragmentBuilder
and into NGContainerFragmentBuilder.

Changes are separated into 3 CLs:

1) Move block_size to NGContainerFragmentBuilder
  block_size was declared in both NGFragmentBuilder, and NGLineBoxFragmentBuilder
2) NGOutOfFlowPositionedDescendant.node must be NGLayoutInputNode,
  not NGBlockNode
  this also required moving UseOldOutOfFlowPositioning into NGLayoutInputNode.
3) move NGFragmentBuilder::AddOutOfFlowChildCandidate and
GetAndClearOutOfFlowDescendantCandidates
to NGContainerFragmentBuilder

Bug: 740993
Change-Id: I98b5b82ba02ebdd7a330d457ad1614d7439e3350
Reviewed-on: https://chromium-review.googlesource.com/736891
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511502}
6 files changed