[webrtc-pc] Define time order of results from getContributingSources() et al (#2189)

jan-ivar has just created a new issue for https://github.com/w3c/webrtc-pc:

== Define time order of results from getContributingSources() et al ==
When returning a sequence, defining its order whenever possible, helps web compat.

For example: A naive implementation might use this for some purpose:
```js
const {audioLevel} = receiver.getSynchronizationSources()[0];
```

...which would work most of the time, when `receiver.getSynchronizationSources().length` is either `1` (good audioLevel) or `0` (`undefined`).

Except once in a blue moon when the SSRC changes for whatever reason, and `receiver.getSynchronizationSources().length == 2` for about 10 seconds.

A browser that returned oldest first would show outdated audio levels for 10 seconds.

I propose we sort the results from newest->oldest based on timestamp (playout time)

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2189 using your GitHub account