Test for postMessage to and from <webview>.

The app is A.
The app has an iframe I1.

The webview is W.
The webview also has an iframe I2.

1) The test can send message to W from A or I1, but sending it
  from I1 would result in event.source being null on the receiving
  side.

2) Once 1) is done, there's a channel to send message from
  webview to the app, so it can
  send message to A from W or I2.
  However sending the message from I2 would result in null
  event.source, similar to 1).