The context is the following proposal to make JSON a subset of
JavaScript: https://github.com/tc39/proposal-json-superset

There’s interest in performing a side investigation to answer the
question of what would happen if we stopped treating U+2028 and U+2029
as `LineTerminator`s *entirely*. (Note that this is separate from the
proposal, which just changes how these characters are handled in
ECMAScript strings.) This is technically a breaking change, and IMHO it
would be wonderful if we could get away with it, but no one really has
any data on whether or not we could. Adding this use counter lets us get
that data.

BUG=v8:6827

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ia22e8db1634df4d3f965bec8e1cfa11cc7b5e9aa
Reviewed-on: https://chromium-review.googlesource.com/693155
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48260}