Bug 253883

Per spec, https://w3c.github.io/media-source/#dom-sourcebuffer-buffered

Step 5,
```
5. If intersection ranges does not contain the exact same range information as the current value of this attribute, then update the current value of this attribute to intersection ranges.
6. Return the current value of this attribute.
```

The current value of this attribute should only be modified if it has changed.

However, currently a copy of the attribute is always returned:
https://searchfox.org/wubkat/rev/7a7d33c945c423639d0e625352d98c40411716f2/Source/WebCore/Modules/mediasource/SourceBuffer.cpp#123-124

```
   // 2. Return a new static normalized TimeRanges object for the media segments buffered.
    return m_private->buffered()->copy();
```

seems to be based on an old spec.