Fix inaccurate comment.

Clearing the StyleResolver does not cause the whole document to be
recalced. The FIXME is still valid I think, just for a different reason.

Review URL: https://codereview.chromium.org/80873004

git-svn-id: svn://svn.chromium.org/blink/trunk@162562 bbb929c8-8fbe-4397-9dbb-9b2b20218538