Move textAlignmentForLine to RenderBlockFlow

We're moving to a world where only RenderBlockFlows hold onto and layout
line box trees.

BUG=302024

Review URL: https://codereview.chromium.org/98123002

git-svn-id: svn://svn.chromium.org/blink/trunk@162952 bbb929c8-8fbe-4397-9dbb-9b2b20218538