UserAgent: Mozilla/5.0 (X11; Linux i686) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/58.0.3029.110 Chrome/58.0.3029.110 Safari/537.36

Steps to reproduce the problem:
1. Call speechSynthesis.speak()
2. 
3. 

What is the expected behavior?
Ability to pass an option to `speak()` which signals that an `ArrayBuffer`, `Blob` or `AudioBuffer` or `data URI` of text passed to `speak()` should be returned from `speak()` call.

What went wrong?
The feature currently does not exist.

Did this work before? No 

Does this work in other browsers? N/A

Chrome version: 58.0.3029.110  Channel: n/a
OS Version: 
Flash Version: 

It is possible to create a recording of audio output from speakers using `navigator.getUserMedia()` and `MediaRecorder`, or by using a library, for example `meSpeak` which has a `rawdata` option. 

We should be able to avoid workarounds and have the ability to pass an option which indicates that audio result of `window.speechSynthesis.speak()` should not be output to speakers, but returned as `ArrayBuffer`, Blob`, `AudioBuffer`, or possibly a `MediaStream`.