[websockets] Correct failure case

[jugglinmike]

The test titled 002.html includes a document which references an
undefined property, asset_equals. Correcting this typo would allow
tests to pass spuriously (that is: when the document is not discarded).
Replace the assertion that was originally intended with
assert_unreached so that user agents which execute this code path
report a test failure.

The test titled 001.html also includes a misspelled reference, but it
does not rely on the value. Remove the reference.

[jugglinmike]

Reviewing more closely, it looks like the existing use of assert_unreached is also incorrect: that seems to be the success case. All browsers are failing the test currently, and three of them fail due to that assertion:

https://wpt.fyi/results/websockets/unload-a-document/002.html?sha=3efff9b&label=master&product=chrome%5Bexperimental%5D&product=edge&product=firefox%5Bexperimental%5D&product=safari%5Bexperimental%5D

So I've added another commit to replace the existing assert_unreached with t.done.

[jugglinmike]

One more bug: the child document uses the uuid function object in the parent as a key to sessionStorage. In another fixup commit, I've replaced it with the intended string value stored in token.

[jugglinmike]

(discovered while researching discrepancies in test results for Safari between WPT's various results collectors)

[jugglinmike]

On further inspection, the expectation as written seems correct to me. The document should be salvageable because its only WebSocket has been closed, so being "made to disappear" during unload should have no effect.

The property references continue to be incorrect, though, so the test cannot pass. I've filed a new pull request that only corrects the references: gh-20190.