[Private Network Access] Fix service worker new script wpt test

When browser requests service worker scripts, it sends a header "Service-Worker".  If the preflight response doesn't safelist it with
"Access-Control-Allow-Headers", the request will fail, which is why the
2 cases of private-network-access/service-worker.https.window.js fails.
This CL fixes it.

Bug: 1239551
Change-Id: Iba1fee1f7384a39c1ce418382dc0efb069853ebf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3858436


Reviewed-by: Weizhong Xia <weizhong@google.com>
Commit-Queue: Jonathan Hao <phao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1041035}