Make GPUPipelineError constructor options optional - #4292

@Kangz That seems reasonable to me. What do you think?

Previews, as seen when this build job started (6a92d46):
WebGPU _{webgpu.idl | Explainer | Correspondence Reference}
WGSL _{grammar.js | wgsl.lalr.txt}

It seems good, but I'd like one of the spec editors @toji or @litherum to approve this.

As raised offline to @foolip, this spec PR needs to be updated as https://gpuweb.github.io/gpuweb/#dom-gpupipelineerror-constructor thinks options is always there:

            1. Set [=this=].{{GPUPipelineError/reason}} to |options|.{{GPUPipelineErrorInit/reason}}.

What was the intention between the spec as written? Maybe both arguments should be required instead of both being optional. In Chrome both are actually required, as calling new GPUPipelineError('bla') throws "TypeError: Failed to construct 'GPUPipelineError': The provided value is not of type 'GPUPipelineErrorInit'."

The reason attribute of the GPUPipelineError needs to be set to one of the valid enums, which I assume is the reason why the second arg is required even though it's nonsensical following a optional arg.

We could declare one of the enum values to be the default (probably "validation"?) and make the dict optional too, but I think it ultimately makes more sense to just make the message required. Devs can always pass empty string if there's really no message to be set.

I found #3709 which points to whatwg/webidl#1211.
We may want to wait for @kainino0x's return to understand how we can fix this.

Thanks for the background on that, Kai! Still looks strange to me but it's pretty clear that the WebIDL spec expects this kind of usage based on the linked example.