That will allow for adding private symbols to JSProxies in a follow-up
change

BUG=chromium:571365
R=neis@chromium.org,verwaest@chromium.org,rossberg@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1575423002

Cr-Commit-Position: refs/heads/master@{#33241}