Update Web Speech API externs

https://webaudio.github.io/web-speech-api/ has some new features.

Eg:
```
// recognition parameters
attribute boolean processLocally;

// methods to drive the speech interaction
undefined start(MediaStreamTrack audioTrack);
static Promise available(SpeechRecognitionOptions options);
static Promise install(SpeechRecognitionOptions options);
```

Is it possible add it?

Thanks!