Spec: make ResizeObserverEntry private

[atotic]

ResizeObserverEntry constructor should not be publicly accessible.

https://bugs.chromium.org/p/chromium/issues/detail?id=674520

[RByers]

Aren't there scenarios where some library may want to synthesize these? Filed w3ctag/design-principles#51 to discuss the general principle.

[atotic]

I had no strong reasons for not including the constructor, it the was the general rule: "do not write code you won't use".
If there is a webapi rule "constructors are exposed by default", I am happy to implement it.

[tabatkins]

Yes, there is a general rule that constructors should be available unless there's good reasons to avoid them (for example, if a valid object requires private browser state, which the user can't reasonably access).