Workers: Avoid synchronously waiting on main thread during startup

In order to make sure that task runners are available on worker
threads we used a semaphore to synchronously wait on the main thread
until workers are actually done setting up task queues.

We see this being a reason for main thread being stalled on JetStream2
bomb-workers benchmark.

The CL switches to using an initialization barrier and explicitly
guards the two relevant internal call sites with it. This avoids the
main thread wait.

Bug: 399391551
Change-Id: I33c49b0bf1339d7ce97eabf5904618b5840428e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6382100
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1436873}