[LayoutNG] Remove checks for layout_box_ inside NGBlockNode.

We now always have one, remove branches. Also removed DCHECKs as its DCHECK'd
in the constructor.

BUG=635619

Review-Url: https://codereview.chromium.org/2749643003
Cr-Commit-Position: refs/heads/master@{#456552}
1 file changed