Expose API for the progress indicator

[annevk]

Per discussion with @domenic this seems in scope for HTML, but we'll need implementation interest beyond Mozilla.

See whatwg/fetch#19 for background. whatwg/fetch#19 (comment) by @gaearon still seems like a reasonable API.

[ojanvafai]

I think Chromium would generally like to see this, but consider it relatively low priority. If there were an API designed, we'd likely implement it at some point.

Also, like @igrigorik said in the referenced XHR issue, it wouldn't work to have a boolean indicator in browsers that show an actual progress bar (e.g. Chrome on Mobile, Safari on Desktop, etc.).