This CL makes ThreadManager::InitThread *the* place that's responsible
for initializing metadata for a new thread, and ensures that all new
threads actually go through there. This was previously not the case,
and e.g. test-lockers/LockerUnlocker exposed a case where some threads
were trying to use another thread's simulator instance because the
ThreadLocalTop on the Isolate was in inconsistent state.

Change-Id: I302c643f420457f6ba73897fd45eb87969e1331c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1781688
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63527}