[LayoutNG] Remove style_ and layout_result_ members on NGBlockNode.

They aren't really needed now. Eventually we'll want NGBlockNode to just
be a POD around the layout object to remove overhead.

After this we can transition NGBlockNode to just be a POD (NGInlineNode
needs a bit more work).

BUG=635619

Review-Url: https://codereview.chromium.org/2726233003
Cr-Commit-Position: refs/heads/master@{#454689}
3 files changed