Checks whether the engine is currently speaking. On Mac OS X, the result
is true whenever the system speech engine is speaking, even if the speech
wasn't initiated by Chrome.
Speaks text using a text-to-speech engine.
utterance The text to speak, either plain text or a complete,
well-formed SSML document. Speech engines that do not support SSML will
strip away the tags and speak the text. The maximum length of the text
is 32,768 characters.
options The speech options.
returns Called right away, before speech finishes. Check
runtime.lastError to make sure there were no errors. Use
options.onEvent to get more detailed feedback.
Stops any current speech and flushes the queue of any pending utterances.
In addition, if speech was paused, it will now be un-paused for the next
call to speak.