Freeze global prototype chain per WebIDL

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/immutable-prototype-serviceworker.js

+function prototypeChain(global) {
+  let result = [];
+  while (global !== null) {
+    let thrown = false;
+    let next = global.__proto__;
+    try {
+      global.__proto__ = {};
+      result.push('mutable');
+    } catch (e) {
+      result.push('immutable');
+    }
+    global = next;
+  }
+  return result;
+}
+
+self.onmessage = function(e) {
+  e.data.postMessage(prototypeChain(self));
+};