[FastTextAutosizer] Add speculative assertions around cluster widths

This patch adds some speculative assertions around cluster width
calculations to try to locate the source of crashes.

BUG=369485

Review URL: https://codereview.chromium.org/265793014

git-svn-id: svn://svn.chromium.org/blink/trunk@173257 bbb929c8-8fbe-4397-9dbb-9b2b20218538