onerror callback should fire if a frame is destroyed before a reply to an outgoing message is received

[Chris Coulson]

Calling WebFrame.sendMessage() returns a request object that is owned by the embedder. The embedder needs to keep this alive until receiving a response to the message. There is currently no trivial way to detect if If the corresponding frame disappears before a response to the message has been sent, at which point, we no longer need to keep the request object around.

If the frame is destroyed, we should really call the onerror handler to notify the embedder of this

[Chris Coulson (chrisccoulson)]

Actually, we do generate an error from the renderer side if a message isn't delivered, but we don't if a message is delivered and then the frame goes away before a reply is sent.