Ensure ParentObjectDestroyed() is in the same sequence of initialization

To understand if crbug.com/409059706#comment19 is true, let me add a
sequence checker to ensure DedicatedWorker::Dispose() is executed with
the same sequence with the worker thread initialization.

Bug: 409059706
Change-Id: I123f989fb946f7b46e65ac7b46f274c36a081963
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6456965
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Keita Suzuki <suzukikeita@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Auto-Submit: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1446916}