Though these are in essence plain objects that need no refcounting,
the presence of a NamedGetter on GleanLabeled means we need to register
everything with the wrappercache.
Paths not taken:
* Making a common utility for Maybe<T> -> dom::Nullable<T>
* GleanEvent::Record taking a dictionary instead of object
* TimerId as opaque Web IDL
* GleanDatetime accept JS Date as well as return it
(( Should make decent follow-ups ))
Differential Revision: https://phabricator.services.mozilla.com/D186267
217642a103