We'll use one generic isolate for all number fuzzers. The previous
deopt isolate will be deleted, once the infra hooks have changed.

Bug: v8:6917
Change-Id: I809613548cd935a0208febb002572cc063697324
Reviewed-on: https://chromium-review.googlesource.com/763534Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49297}