Freeze global prototype chain per WebIDL

third_party/WebKit/LayoutTests/security/immutable-prototype.js

Index: third_party/WebKit/LayoutTests/security/immutable-prototype.js
diff --git a/third_party/WebKit/LayoutTests/security/immutable-prototype.js b/third_party/WebKit/LayoutTests/security/immutable-prototype.js
new file mode 100644
index 0000000000000000000000000000000000000000..7af5e079fae3862d850a69a7e5da30ebd24918e3
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/security/immutable-prototype.js
@@ -0,0 +1,16 @@
+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;
+}
+