The current spec defines that the default value of this option is "omit", but there is a spec discussion that we change it to "same-origin":
https://github.com/whatwg/html/pull/3656

This discussion is tracked by a separate crbug  issue 838540 .

In this  issue 843875  I'll add tests based on the current spec, and then change the default value in  issue 838540 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f844404708be03a8bff069cf742cf43024a78420

commit f844404708be03a8bff069cf742cf43024a78420
Author: Hiroki Nakagawa <nhiroki@chromium.org>
Date: Fri May 18 14:44:40 2018

Worker: Add credentials tests for dedicated workers

This CL adds web-platform-tests for the "credentials" option of WorkerOptions:
https://html.spec.whatwg.org/multipage/workers.html#workeroptions

The current spec defines that the default value of this option is "omit", but
there is an ongoing spec discussion to change it to "same-origin":
https://github.com/whatwg/html/pull/3656

This CL adds the tests based on the current spec, and a subsequent CL will
update them based on the decision.

Bug:  843875 
Change-Id: I50eb0c7971587b9d84865498d67abef8ed2d8fc6
Reviewed-on: https://chromium-review.googlesource.com/1063524
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559888}
[add] https://crrev.com/f844404708be03a8bff069cf742cf43024a78420/third_party/WebKit/LayoutTests/external/wpt/workers/modules/dedicated-worker-options-credentials.html
[add] https://crrev.com/f844404708be03a8bff069cf742cf43024a78420/third_party/WebKit/LayoutTests/external/wpt/workers/modules/dedicated-worker-options-credentials.html.headers
[add] https://crrev.com/f844404708be03a8bff069cf742cf43024a78420/third_party/WebKit/LayoutTests/external/wpt/workers/modules/resources/credentials.py