Spec issue for worklets: https://github.com/w3c/css-houdini-drafts/issues/756

The worklets spec was changed. I'll update the implementation and tests.

Created an entry for worklets on the feature dashboard:
https://www.chromestatus.com/feature/6383899018002432

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2c792e62c984f1effa9067c18e8653dd22e9f6fb

commit 2c792e62c984f1effa9067c18e8653dd22e9f6fb
Author: Hiroki Nakagawa <nhiroki@chromium.org>
Date: Thu May 10 06:37:56 2018

Worklet: Change the default credentials mode from 'omit' to 'same-origin'

According to a recent spec change, this CL changes the default credentials mode
on WorkletOptions from 'omit' to 'same-origin'.

Spec issue: https://github.com/w3c/css-houdini-drafts/issues/756
Feature dashboard: https://www.chromestatus.com/feature/6383899018002432

Bug:  838540 
Change-Id: I341f9c61e11387fe7192d75e616b4003fa36128f
Reviewed-on: https://chromium-review.googlesource.com/1051307
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557467}
[modify] https://crrev.com/2c792e62c984f1effa9067c18e8653dd22e9f6fb/third_party/WebKit/LayoutTests/external/wpt/worklets/resources/credentials-tests.js
[modify] https://crrev.com/2c792e62c984f1effa9067c18e8653dd22e9f6fb/third_party/blink/renderer/core/workers/worklet_options.idl

For easier feature tracking, let me narrow down the scope of this issue only to worklets because a change for worklets was in M68 and could affect real applications, while a change for workers hasn't been landed yet and module loading for workers are still behind the flag. I'll file a separate issue for workers.