Version: 51.0.2704.106, probably M50 and M49
OS: Android

What steps will reproduce the problem?
Run the following:
var options = [
  { initDataTypes: ['cenc'],
    audioCapabilities: [ {
      contentType: 'audio/mp4; codecs="mp4a.40.2"',
      robustness: 'SW_SECURE_CRYPTO'
    }]
  }   
];
navigator.requestMediaKeySystemAccess('com.widevine.alpha', options)
.then(mediaKeySystemAccess => console.log('success'))
.catch(e => console.log(e));

What is the expected output?
"success"

What do you see instead?
"DOMException: None of the requested configurations were supported.
Promise {[[PromiseStatus]]: "resolved", [[PromiseValue]]: undefined}"

Please use labels and text to provide additional information.
This works on Linux 51.0.2704.106 and in M52 and M54 on Android.

If `robustness` is omitted or the empty string, the call succeeds.