The race occurs when the write to instance_ happens after the static
thread-local initialization. The change simply moves the write into
CagedHeap's ctor.

Bug: chromium:1325007
No-Try: true
Change-Id: I55b6e286f311c9ddb4f3f89d12f7f9437b96cc58
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3702334
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81108}