Revert of Get rid of UnsafeCurrent in Sampler (patchset #12 id:220001 of https://codereview.chromium.org/1858143003/ )

Reason for revert:
[Sheriff] Breaks tsan:
https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20TSAN/builds/8999

Original issue's description:
> Get rid of UnsafeCurrent in Sampler
>
> Currently we are using UnsafeCurrent in async signal handler to acquire the
> isolate of VM thread, but we want to get rid of that since it prevents V8 from
> being thread agnostic.
>
> This patch replaces UnsafeCurrent with a static map, where we store a map of
> samplers for threads, and makes it accessible by signal handler.
>
> BUG=v8:4889
> LOG=n
>
> Committed: https://crrev.com/62fb4775fea0d56d8a175baf1d902213f6752168
> Cr-Commit-Position: refs/heads/master@{#35541}

TBR=jochen@chromium.org,alph@chromium.org,fmeawad@chromium.org,yangguo@chromium.org,lpy@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4889

Review URL: https://codereview.chromium.org/1897673002

Cr-Commit-Position: refs/heads/master@{#35545}