Editorial: Update use of WebIDL "invoke a callback function"

[jeremyroman]

The only use which is not promise-returning is the "start" algorithm, which should rethrow the exception (so it can in turn be rethrown from the ReadableStream constructor.

Part of whatwg/webidl#1425.

• At least two implementers are interested (and none opposed): n/a (no normative change)
• Tests are written and can be reviewed and commented upon at: n/a (no normative change)
• Implementation bugs are filed: n/a (no normative change)
• MDN issue is filed: n/a (no normative change)
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

[ricea]

lgtm