Type mismatch in colorSpace parameter to Convert to RGB frame

[jyasskin]

https://w3c.github.io/webcodecs/#dom-videoframe-copyto takes a colorSpace of type PredefinedColorSpace, an enum of "srgb" and "display-p3". It passes it to https://w3c.github.io/webcodecs/#videoframe-convert-to-rgb-frame, which assigns it to https://w3c.github.io/webcodecs/#dom-videoframe-color-space-slot of type VideoColorSpace, which is a whole constructible interface. There needs to be a conversion of some sort in there.

I think this was introduced in #754. cc/ @Djuffin