This was functionally equivalent to SymmetricKey::key(), and also
returned a always true status code. Switching to key() allowed for
the simplification of downstream functions.

R=alemate@chromium.org, alokp@chromium.org, davidben@chromium.org, rogerta@chromium.org, stanisc@chromium.org

Review-Url: https://codereview.chromium.org/2934893003 .
Cr-Commit-Position: refs/heads/master@{#480868}