These are cases that are implementing the "convert an exception to a Promise"
steps of the Web IDL spec. Typically the exception is thrown in the current
compartment; the Promise returned should simply match that. Otherwise we can
end up with a situation in which the promise doesn't actaully have access to
its rejection value, which will cause problems if someone uses then() on the
promise: the return value of the then() call will get a sanitized exception
instead of the real one.
We _could_ try to match the actual compartment of the exception, in theory.
But it's not clear why this would be preferable to using the current
compartment, even if there were cases in which the exception _doesn't_ match
the current compartment. Which there likely are not.
MozReview-Commit-ID: Ac2BHIHxfvY
Without this change, we could do a "short" get of a string (e.g. from
XMLHttpRequest), then do another get that returns the same stringbuffer but a
longer length. But we wouldn't notice, hit our cache, and return the same,
too-short, external string. The site would not see the new data it should have
seen.
