Ignore messages other than from iframe parent

[lukaw3d]

Malformed messages from extensions throw errors (though download still works). This filters out messages, except those coming from iframe parent.

To reproduce errors:

• Install MetaMask extension
• Open https://jimmywarting.github.io/StreamSaver.js/examples/fetch.html
• Click "Start" button
• See errors in browser console

Uncaught TypeError: [StreamSaver] You didn't send a messageChannel    at onMessage (mitm.html?version=2.0.0:66)
Uncaught TypeError: [StreamSaver] You didn't send a messageChannel    at onMessage (mitm.html?version=2.0.0:66)
Uncaught (in promise) TypeError: [StreamSaver] You didn't send a messageChannel    at onMessage (mitm.html?version=2.0.0:66)
Uncaught TypeError: [StreamSaver] You didn't send a messageChannel    at onMessage (mitm.html?version=2.0.0:66)
Uncaught TypeError: [StreamSaver] You didn't send a messageChannel    at onMessage (mitm.html?version=2.0.0:66)

[AmilKey]

@jimmywarting do you have plans to merge? this PR also works for other extensions