[w3c/webcomponents] Clarify upgrading element's properties (#660)

I've come across what I think is a bug in CE Upgrades, however I'm not sure if it's intended, and apologies if I'm missing something in the spec.

Basically, if my Custom Element definition uses a getter and setter, these will be overwritten with the corresponding property _value_ that exists on the element when that element is upgraded, losing those getters and setters.

See [this JSBin](http://jsbin.com/ribused/2/edit?js,console) - the first element after upgrading retains the initial `foo` value, and the getters and setters are lost. The second element which is created after the CE definition is registered, demonstrates expected functionality.

Reading the [upgrades definition](https://html.spec.whatwg.org/multipage/custom-elements.html#upgrades) I can't quite figure out intended behaviour? Not sure if in fact I should just be reporting this to Blink / WebKit

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