[FastTextAutosizer] Remove comment about storing a fingerprint stack

This comment is no longer true (it may have never been either). The
fingerprint logic is fairly cheap because we cache the parent hash.

TBR=skobes
NOTRY=true

Review URL: https://codereview.chromium.org/207193002

git-svn-id: svn://svn.chromium.org/blink/trunk@169708 bbb929c8-8fbe-4397-9dbb-9b2b20218538