[whatwg/dom] `customelement.blur()` should cancel all the focus in ShadowDOM? (#1008)

example:
```js
class MyElement extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({mode: 'open'});
    this.shadowRoot.innerHTML = '<div tabindex=1>foucesable</div>';
  }
  
  connectedCallback() {
    this.shadowRoot.querySelector('div').focus();
  }
}
customElements.define('my-ele', MyElement);
document.body.append(new MyElement());

document.querySelector('my-ele').blur();
document.body.append(`current active element: ${document.activeElement.tagName}`);
```

Firefox: current active element: MY-ELE 
Chrome: current active element: BODY
Safari: current active element: BODY

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