This CL refactors the lifetime management of the v8::Worker C++ object
and in the process lifts the 100 maximum worker limitation. To do this,
it uses a Managed<v8::Worker> heap object and attaches the managed to
the API worker object.

R=mstarzinger@chromium.org
BUG=v8:9524

Change-Id: I279b7aeb6645a87f9108ee6f572105739721cef4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1715453
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62932}