Reset main_thread_local_isolate_ only after Heap::TearDown was executed.
main_thread_local_isolate_ is still needed in there for e.g.
HandleBase::IsDereferenceAllowed in MemoryMeasurement.

Bug: chromium:1150867, v8:10315
Change-Id: Ia1ebfd561b7a3ab2d346f0c17b239f75ad77471f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2549969Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71310}