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

[RndUsername]

+1 this fixed my issues when self hosting the mitm script

[PekingSpades]

Note that this workaround does not work across domains.
Cross-domain means that mitm.html and the page using streamSaver.js are not on the same domain. For this project, if the page using streamSaver.js is under HTTP, it will inevitably use the third-party hosted mitm.html.
If you are using it under HTTP, it is not recommended that you merge the code above.

There is, of course, a compromise. When using mitm.html under the same site, ignore messages from other sources. When hosting mitm.html under another domain, keep it the same.