sendResponse property
Function to call (at most once) when you have a response. The argument
should be any JSON-ifiable object. If you have more than one onMessage
listener in the same document, then only one may send a response. This
function becomes invalid when the event listener returns, unless you
return true from the event listener to indicate you wish to send a
response asynchronously (this will keep the message channel open to the
other end until sendResponse is called).
Implementation
final JSFunction sendResponse;