Move generatesLineBoxesForInlineChild to RenderBlockFlow

Line boxes should never be handled by the more generic RenderBlock,
but always by RenderBlockFlows. Pulling this functionality down,
which also uncovered some simplification/type tightening in
RenderListItem.

BUG=302024

Review URL: https://codereview.chromium.org/62993002

git-svn-id: svn://svn.chromium.org/blink/trunk@161497 bbb929c8-8fbe-4397-9dbb-9b2b20218538