isConfigSupported: definition of "invalid"

The behavior of isConfigSupported API for unsupported configurations is different on Chromium and Safari.

The specification says (for Audio and Video encoders and decoders):

"If config is not a valid.... return a promise rejected with TypeError."

However, the definition of "invalid" appears to differ between implementations.

On Chromium, when an unsupported scalabilityMode value is provided, the promise resolves with supported set to "false", but on Safari, the promise is rejected.

Live example: https://webrtc.internaut.com/wc/isup2/

A snippet:

async function modeProperties(mode, enc, config) {
  config.scalabilityMode = mode;
  if (enc == 'true') {
    // check whether the encoder supports the configuration
    try {
      const encoderSupport = await VideoEncoder.isConfigSupported(config);
      if (encoderSupport.supported) {
        addToEventLog('For encode ' + preferredCodec + ' ' + mode + ' is supported');
      } else {
        addToEventLog('For encode ' + preferredCodec + ' ' + mode + ' is NOT supported');
        //addToEventLog('Config details:\n' + JSON.stringify(encoderSupport.config));
      }
    } catch (e) {
     // Safari will end up here for unsupported scalabilityMode values, Chromium will not
      addToEventLog('For encode ' + preferredCodec + ' ' + mode + ' is considered INVALID');
    }
  } else {
    // check whether the decoder supports the configuration
    try {
      const decoderSupport = await VideoDecoder.isConfigSupported(config);
      if (decoderSupport.supported) {
        addToEventLog('For decode ' + preferredCodec + ' ' + mode + ' is supported');
      } else {
        addToEventLog('For decode ' + preferredCodec + ' ' + mode + ' is NOT supported');
      }
    } catch (e) {
     // Safari will end up here for unsupported scalabilityMode values, Chromium will not
      addToEventLog('For decode ' + preferredCodec + ' ' + mode + ' is considered INVALID');
    }
  }
}

Start with the specification text quoted in the issue and reproduce the live example at https://webrtc.internaut.com/wc/isup2/ in Chromium and Safari. Compare how VideoEncoder.isConfigSupported and VideoDecoder.isConfigSupported handle unsupported scalabilityMode values; done means the specification or implementation discrepancy has a decided resolution.