Define getters for read-only attributes (Was: interface NavigatorUAData should be a dictionary)

[domenic]

As this is not a class with a useful constructor, data, and behavior packaged together, but instead a simple holder for data, it would be better as a dictionary.

[domenic]

This would also solve the problem where the getter algorithms for each field of NavigatorUAData are not defined, and instead https://wicg.github.io/ua-client-hints/#dom-navigatorua-getuseragent somehow sets the value of readonly getter fields.

[yoavweiss]

We've reached the conclusion that it might be better off to remove the Promise from getUserAgent()'s return value, and add it on single attributes. I believe that design is not possible with a dictionary, so we would need to properly define the getters.

[yoavweiss]

#48 is the PR in which we're moving the promises.
There may be work left after it though (e.g. explicitly setting the getters on the read only attributes)

[yoavweiss]

I believe this was addressed by #70.