[w3c/webcomponents] observedAttributes should be converted to ASCII lowercase (#838)

Since both `Element.setAttribute` and `Element.getAttribute` is converting the Attribute name to ASCII lowercase by default,  the `sequence<DOMString>` returned from `observedAttributes` should also be converted the same way.

If we take the flag as an example:
```HTML
<flag-icon selectedCountry="no"></flag-icon>
``` 
And then do this
```javascript
class FlagIcon extends HTMLElement {
    constructor() {
        super();
        //...
    }

    static get observedAttributes() { return ["selectedCountry"]; }

    attributeChangedCallback(name, oldValue, newValue) {
        //...
    }

    //...
}

customElements.define('flag-icon', FlagIcon);
```

This example will not work since the the name in the tag is converted to ASCII lowercase, and the observedAttribute list is not.

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