This test currently assume that cloning a COW will fail. Once we always go over
security wrappers, this isn't true anymore, because COWs silently deny
disallowed property gets. So it makes sense to rewrite the test so that we can
see what actually makes it through the structured clones. Promises galore.
We also take out the buggy Blob/FileList stuff and replace it with ImageData,
which does a bonafide clone in NS_DOMWriteStructuredClone instead of the weird
pointer swapping we do in nsGlobalWindow.cpp (baku is in the process of cleaning
that up, but it's not done yet).
