Reland: service worker: Add virtual tests for on-the-main-thread script fetch

The previous CL was reverted because it missed some test expectations.
Added missing expectations.

Original description:
> On-the-main-thread script fetch is disabled by default. Add virtual
> tests for on-the-main-thread script fetch to have better test
> coverage until we ship off-the-main-thread script fetch.

Change-Id: I03d9dd44957f3e58d984a76200ff12a41ae28d6f
Bug: 967272
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1675645
Auto-Submit: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672741}
4 files changed