Tighten up RootInlineBox creation to RenderBlockFlow

All line layout work should be relegated to RenderBlockFlow,
not RenderBlock. This is just another baby step towards
being able to remove the line box tree pointer from RenderBlock.

BUG=302024

Review URL: https://codereview.chromium.org/93513012

git-svn-id: svn://svn.chromium.org/blink/trunk@164603 bbb929c8-8fbe-4397-9dbb-9b2b20218538