Isolate::thread_id() may be invoked on a background thread to perform
a check if the thread is the main thread.

This also removes the copy constructor of ThreadLocalTop and clears
its fields explicitly.

Change-Id: I2207bf1ebd655805841fce9f23d23cff7be8f13b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2537693Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71177}