concurrent marking.

The function should use relaxed store similar to other JSObject setters.

BUG=chromium:694255

Change-Id: I032f0763a5f2420d120bce976533aa0007868b97
Reviewed-on: https://chromium-review.googlesource.com/565573Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46535}