This patch makes ECMAScript a syntactic superset of JSON by allowing
U+2028 and U+2029 in string literals.

Proposal repo: https://github.com/tc39/proposal-json-superset

Bug: v8:7418
Change-Id: I7ef4ae6d85854ebc44a66e0eaf789814576832b7
Reviewed-on: https://chromium-review.googlesource.com/921228Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51313}