browsing data: Simplify threading of service worker test utils.

The "service worker core thread" is just the UI thread, so
RunOrPostTaskOnThread just invokes the function directly.

Bug: 1138155
Change-Id: Id004877a05f76942b9e1b93310bb351131c15523
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3144254


Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918728}