Set the test-widget (iframe) height according to its content height

[notetiene]

Using an iFrame instead of WebComponents for the test-widget introduced an unexpected behaviour, namely:

• The Height of the test-widget window can only be fixed. It can't grow as content is added in the iFrame. The workaround I found is to calculate the iFrame window's height and pass it to the test-widget container. However, it doesn't seem to work when listening for DOMContentLoaded because when an iFrame is created, it also create a dummy document. It may be possible to emulate that event with a MutationListener.

[notetiene]

As of the firefox branch, this is now fixed.