[LayoutNG] A few small cleanups in flex code

 - Don't use legacy when we don't have to
 - Store NGBlockNode instead of NGLayoutInputNode

Bug: 845235
Change-Id: I99e48f7f69e4f53f0391e2cd542e117b501cb7d6
Reviewed-on: https://chromium-review.googlesource.com/c/1379061
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616907}
NGFlexLayoutAlgorithm::NGFlexLayoutAlgorithm(NGBlockNode node,
                                             const NGConstraintSpace& space,
                                             const NGBreakToken* break_token)
    : NGLayoutAlgorithm(node, space, ToNGBlockBreakToken(break_token)) {
  container_builder_.SetIsNewFormattingContext(space.IsNewFormattingContext());
}

NGFlexLayoutAlgorithm::NGFlexLayoutAlgorithm(NGBlockNode node,
                                             const NGConstraintSpace& space,
                                             const NGBreakToken* break_token)
    : NGLayoutAlgorithm(node, space, ToNGBlockBreakToken(break_token)) {
  container_builder_.SetIsNewFormattingContext(space.IsNewFormattingContext());
}
