Allows the MessagePort to begin receiving messages.
This method has no parameters.
This method does not return a value.
The start method is called implicitly by when an event handler function is assigned to the onmessage property. In Internet Explorer 10, the start method is also called implicitly when a message event is registered using the addEventListener method.
For more info, see MessageChannel for a working messaging demo.
Build date: 11/20/2012