Move fitBorderToLinesIfNeeded and most of deleteLineBoxTree into RenderBlockFlow.

fitBorderToLinesIfNeeded can only be called on block flows, so it can be moved
altogether.

deleteLineBoxTree can have its logic moved, leaving (for now) only an ASSERT
declaring that it wouldn't have done anything anyway for now.

BUG=393945, 302024

Review URL: https://codereview.chromium.org/404993002

git-svn-id: svn://svn.chromium.org/blink/trunk@178577 bbb929c8-8fbe-4397-9dbb-9b2b20218538