Allocation observers are not thread-safe, so the allocation steps should
be invoked only on the main thread.

Bug: v8:10536
Change-Id: I90e809fa230d18cce135f4bea67b62ef616ca593
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2207189Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67891}