Layering: change SetImmutablePrototype to use [[GetPrototypeOf]] (#841)

SetImmutablePrototype is meant for use by a variety of objects beyond the ECMA-262 spec. Some of those objects do not have a [[Prototype]], or have a [[Prototype]] but use custom [[GetPrototypeOf]] logic to act is they don't sometimes. (Respectively: HTML's WindowProxy and Location objects.) As such, SetImmutablePrototype should look up the prototype using [[GetPrototypeOf]] instead of trying to look at the possibly-nonexistant [[Prototype]] slot directly.

Some background:

* web-platform-tests/wpt#5015 (comment)
* whatwg/html#2400

spec.html

@@ -8451,7 +8451,7 @@ <h1>SetImmutablePrototype ( _O_, _V_ )</h1>
         <p>When the SetImmutablePrototype abstract operation is called with arguments _O_ and _V_, the following steps are taken:</p>
         <emu-alg>
           1. Assert: Either Type(_V_) is Object or Type(_V_) is Null.
-          1. Let _current_ be _O_.[[Prototype]].
+          1. Let _current_ be ? _O_.[[GetPrototypeOf]]().
           1. If SameValue(_V_, _current_) is *true*, return *true*.
           1. Return *false*.
         </emu-alg>