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

Review URL: https://codereview.chromium.org/1858143003

Cr-Commit-Position: refs/heads/master@{#35541}