[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}
LayoutUnit NGFlexLayoutAlgorithm::MainAxisContentExtent(
    LayoutUnit sum_hypothetical_main_size) {
  if (Style().IsColumnFlexDirection()) {
    return ComputeBlockSizeForFragment(
               ConstraintSpace(), Style(),
               sum_hypothetical_main_size + (borders_ + padding_).BlockSum()) -
           border_scrollbar_padding_.BlockSum();
  }
  return content_box_size_.inline_size;
}

LayoutUnit NGFlexLayoutAlgorithm::MainAxisContentExtent(
    LayoutUnit sum_hypothetical_main_size) {
  if (Style().IsColumnFlexDirection()) {
    return ComputeBlockSizeForFragment(
               ConstraintSpace(), Style(),
               sum_hypothetical_main_size + (borders_ + padding_).BlockSum()) -
           border_scrollbar_padding_.BlockSum();
  }
  return content_box_size_.inline_size;
}
