The ES spec has been updated to include this legacy syntax in Annex B:
https://tc39.github.io/ecma262/#sec-initializers-in-forin-statement-heads

R=neis@chromium.org
BUG=v8:4942

Review-Url: https://codereview.chromium.org/2407863003
Cr-Commit-Position: refs/heads/master@{#40189}