Auto merge of #12980 - jdm:proxychanges, r=nox

Fix ridiculous DOM proxy getter performance

This implements the missing shadowing checks that were causing us to take many slow paths when dealing with proxy objects. Verified by running `tests/html/binding_perf.html` before and after and observing a 12x improvement.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12357
- [X] These changes do not require tests because we can't test performance yet.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12980)
<!-- Reviewable:end -->

tests/html/bindings_perf.html

@@ -1,6 +1,7 @@
 <button onclick="void_method()">measure void method</button>
 <button onclick="int_getter()">measure int getter</button>
 <button onclick="firstChild_getter()">measure firstChild getter</button>
+<button onclick="proxy_firstChild_getter()">measure proxy firstChild getter</button>
 <script>
 var t = 'TestBinding' in window ? (new TestBinding()) : (new TextEncoder());
 function void_method() {
@@ -33,4 +34,15 @@ function firstChild_getter() {
   var stop = new Date();
   console.log('firstChild getter: ' + ((stop - start) / count * 1e6) + 'ns');
 }
+
+function proxy_firstChild_getter() {
+  var n = document;
+  var start = new Date();
+  var count = 1000000;
+  for (var i = 0; i < count; i++) {
+    var a = n.firstChild;
+  }
+  var stop = new Date();
+  console.log('proxy firstChild getter: ' + ((stop - start) / count * 1e6) + 'ns');
+}
 </script>