UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.77 Safari/537.36

Steps to reproduce the problem:
1. Use DOMParser to create a DOM from XML with a namespace declared on the root element.
2. Use setAttributeNS to create an attribute on a child node using the URI (but not prefix) from the declared namespace prefix.
3. Use XMLSerializer to serialize the DOM.

```
let doc = (new DOMParser).parseFromString('<r xmlns:xx="uri"><b/></r>', 'text/xml');
doc.querySelector('b').setAttributeNS('uri', 'foo', 'bar');
(new XMLSerializer).serializeToString(doc);
```

What is the expected behavior?
The namespaced attribute on the child node uses the namespace prefix declared on the root.

```
<r xmlns:xx="uri"><b xx:foo="bar"/></r>
```

What went wrong?
Chrome generates a new namespace prefix for the existing URI unless setAttribute() is called using the existing namespace prefix.

```
<r xmlns:xx="uri"><b xmlns:ns3839401="uri" ns3839401:foo="bar"/></r>
```

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 70.0.3538.77  Channel: stable
OS Version: 10.0
Flash Version: 

This works correctly on Firefox and Safari.
This test is hosted at http://phrogz.net/tmp/xmlserializertest.html
 

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