customElements.define isn't checking disabledFeatures and formAssociated when expected

[pshaughn]

WPT custom-elements/CustomElementRegistry has various tests that set up call-trace proxies around a custom class and then call customElements.define to see which fields are accessed. In general, disabledFeatures and formAssociated are expected to be accessed but aren't. A quick grep of servo's codebase suggests these strings might not even be in it.

https://html.spec.whatwg.org/multipage/custom-elements.html#custom-elements-api specifies getting them, in substeps 14-6 and 14-12 of the define method.