Remove 'paginated line width' from RootInlineBox.

It was set but never used. Probably a relic from CSS regions support.

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/716213002

git-svn-id: svn://svn.chromium.org/blink/trunk@185233 bbb929c8-8fbe-4397-9dbb-9b2b20218538