Change credentials mode default

[annevk]

We shouldn't use "omit" as a default. We should use "same-origin" for web developer convenience. See whatwg/html#3656. (Note that fetch() already changed to no longer have this as a default.)

Fixing this will also require changing https://github.com/w3c/web-platform-tests/blob/master/worklets/resources/credentials-tests.js.

cc @nhiroki

[nhiroki]

Chromium issue: https://crbug.com/838540

Should we remove credentials field from WorkletOptions like changes for the worker spec, or remove WorkletOptions entirely?

[nhiroki]

I'll update the WPTs and Chromium implementation after the spec change is clarified.

[annevk]

@nhiroki note that @domenic suggested we keep credentials and just flip the default. I guess I'm good either way, though currently we don't offer "omit" outside of fetch().

[nhiroki]

@annevk Thank you! I forgot the domenic's comment in the PR :) I'll change them in that way.

[nhiroki]

Fixed Chromium impl and WPTs. Can you close this issue?

[annevk]

And the specification changed with #757. Thanks!