SVGImageElement.crossOrigin is specified.

Three problems here:
- First, this is specified, so the comment is incorrect.
- Second, should_throw isn't supposed to be a function, it's not called below.
It needs to be a value that's then tested in a condition.
- Third, if an implementation doesn't support crossOrigin on SVGImageElement,
this test should fail as to not mask a non-conformity and mask a possible
incompatibility.

Depends on D181377

Differential Revision: https://phabricator.services.mozilla.com/D181378

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1839169
gecko-commit: 2b4f8ad0cecd1e9ce287bf572fd28b8b94d91717
gecko-reviewers: chunmin