This was causing GC-related crashes on various websites, most
prominently on any site that contains embedded YouTube videos. The issue
can be reproduced by going to any YouTube video, using the _Share_
button below it and pasting the embed code into an empty HTML file and
loading it through localhost.
This is technically a regression from
89dbdd3411 in that the problem became
visible with that commit. However, there is nothing wrong with the
commit by itself. It just happens that `Origin::is_same_origin_domain()`
prior to that commit was completely bogus and would mistakenly return
true in almost all cases, so the cross-origin code paths were not
exercised.
I am uncertain how to make a automatic test case for this problem, given
the nature of it being GC- and cross-origin-related. So there is no
regression test included in this commit.
The special empty value (that we use for array holes, Optional<Value>
when empty and a few other other placeholder/sentinel tasks) still
exists, but you now create one via JS::js_special_empty_value() and
check for it with Value::is_special_empty_value().
The main idea here is to make it very unlikely to accidentally create an
unexpected special empty value.
