[FastTextAutosizer] Autosize lists in the first layout

This patch fixes a bug where lists were only autosized if there were
two layouts. FastTextAutosizer::inflateListItem is called before the
list item has entered layout and so the width will initially be zero.
The fix is to use the same trick tables use: use the containing block's
width.

BUG=363767

Review URL: https://codereview.chromium.org/275193004

git-svn-id: svn://svn.chromium.org/blink/trunk@173909 bbb929c8-8fbe-4397-9dbb-9b2b20218538