Prevent a use after free in computeBlockPreferredLogicalWidths

This patch fixes a use after free during preferred logical width
calculation when we have autosizing enabled. This is a speculative
local fix to see if more of these regressions pop up; the final fix
will need to address crbug.com/389648 as well.

BUG=385268

Review URL: https://codereview.chromium.org/358163002

git-svn-id: svn://svn.chromium.org/blink/trunk@177152 bbb929c8-8fbe-4397-9dbb-9b2b20218538