fix: make userContext mandatory in browsingContext.CreateResult

[juliandescottes]

The algorithm for browsingContext.create reads as follows:

Let body be a map matching the browsingContext.CreateResult production, with the context field set to traversable’s navigable id and the userContext property set to the user context id of traversable’s associated user context.

Nothing here can lead userContext to be missing so I don't see why the property should be optional?

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[juliandescottes]

Ok I see that this seems to be another follow up of #1046 which not only added userContext to events, but also to the return value of browsingContext.create.

I will close in favor of #1071, I guess we can make it mandatory at the same time as the rest (in the future we should add a note in the spec when we make fields temporarily optional, that would save time ...)