Fix ScriptPromiseResultTracker nits

1. Remove unused member (timeout_interval_).
2. Mark const members as const.
3. Change CHECK into a DCHECK.

Bug: None
Change-Id: If55718cf2c7161c1523857da5f02beff5119296b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4061574


Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Elad Alon <eladalon@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Palak Agarwal <agpalak@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1076890}