[whatwg/dom] Allow attachShadow to HTMLUnknownElement (#517)

I create a element with
```javascript
var toolbar = document.createElement('toolbar')
```
 then call

```javascript
toolbar.attachShadow({mode: 'open'})
```
It throws
```
VM4423:1 Uncaught DOMException: Failed to execute 'attachShadow' on 'Element': This element does not support attachShadow at <anonymous>:1:31
```
but this works:
```javascript
toolbar.createShadowRoot()
```
I don't want to use native custom element there, because it too restrictions, but i want use ShadowDOM there?
Why ShadowDOM V1 droped the support of `HTMLUnknownElement`?


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