Fixes corner case where arrow function ConciseBody expression does not
accept 'in' in productions.

BUG=v8:4472
LOG=N
R=wingo@igalia.com, adamk@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/1407633002

Cr-Commit-Position: refs/heads/master@{#31271}