At the moment the flag is set too late, it is possible that the GC still
tries to post tasks in Isolate::Deinit when the isolate is already
disconnected from the platform, see the referenced bug.

R=ulan@chromium.org

Bug: chromium:810739
Change-Id: Ibcd226cb44cc903f2a46e7cccf682b3938c9d408
Reviewed-on: https://chromium-review.googlesource.com/915942Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51274}