Fix inspector/service-workers/user-agent-override.html for off-main-thread-fetch virtual test by setting experiment flag

Bug: 443374
Change-Id: Ibc1201d66a29709a0e5b1f95ca5a2bbe08f40b4a
Reviewed-on: https://chromium-review.googlesource.com/558891
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490200}
2 files changed