Move pagination struts to RenderBlockFlow.

Only block flows can have pagination struts.

BUG=393945,302024

Review URL: https://codereview.chromium.org/396693002

git-svn-id: svn://svn.chromium.org/blink/trunk@178208 bbb929c8-8fbe-4397-9dbb-9b2b20218538