Enables web developers to incorporate speech synthesis into their web pages. Uses whatever the platform provides for synthesis. Coming in M71, synthesis will have a few small changes. 1. Exposing the following existing interfaces: SpeechSynthesis, SpeechSynthesisVoice, SpeechSynthesisEvent 2. Adding a constructor to SpeechSynthesisEvent 3. Adding and exposing a new interface, SpeechSynthesisErrorEvent.