Document assert_object_equals

[foolip]

I saw it used in https://chromium-review.googlesource.com/c/chromium/src/+/978128 and wasn't sure exactly what it does and didn't find it in http://web-platform-tests.org/writing-tests/testharness-api.html.

Per #2033 some think it should be removed instead, but if it's not deleted it'd be useful to know about the limitations.

[foolip]

Implementation at https://github.com/w3c/web-platform-tests/blob/355c2d77c7ec01ee18fd9c5917cc845a558551e8/resources/testharness.js#L964.

AFAICT, what it does is to check that all enumerable properties on one object are own properties on the other with the same values, recursing into other objects and dealing with cycles.

Because it isn't checked that the inputs are really objects, things like assert_object_equals({}, null), assert_object_equals(1, 2) and assert_object_equals({}, []) all pass because for (x in null) {} and similar doesn't throw an exception.