Add MediaStreamTrack support to the Web Speech API

[evanbliu]

Request for Mozilla Position on an Emerging Web Specification

• Specification title: Add MediaStreamTrack support to the Web Speech API
• Specification or proposal URL (if available): Add MediaStreamTrack support to the Web Speech API spec WebAudio/web-speech-api#118
• Explainer URL (if available): N/A
• Proposal author(s) (@-mention GitHub accounts): @evanbliu
• Caniuse.com URL (optional): N/A
• Bugzilla URL (optional): N/A
• Mozillians who can provide input (optional): @padenot
• WebKit standards-position: Add MediaStreamTrack support to the Web Speech API WebKit/standards-positions#438

Other information

The Web Speech API is a web standard API that allows developers to incorporate speech recognition and synthesis into their web pages. Currently, the Web Speech API uses the user's default microphone as the audio input. MediaStreamTrack support allows websites to use the Web Speech API to caption other sources of audio including remote audio tracks.

[padenot]

We're also positive on this. This is covered in the PR for #1162 since it's so closely related.