devtools: Simplify background services since service worker is on UI.

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

Bug: 1138155
Change-Id: Iedc6629076b78f15e1552472747bbb5ec5ba9d98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3144255


Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918760}