Don't touch the prototype chain to get the private script controller.

third_party/WebKit/Source/bindings/core/v8/PrivateScriptRunner.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "bindings/core/v8/PrivateScriptRunner.h"
 
 #include "bindings/core/v8/DOMWrapperWorld.h"
 #include "bindings/core/v8/ExceptionState.h"
 #include "bindings/core/v8/V8Binding.h"
 #include "bindings/core/v8/V8PerContextData.h"
@@ skipping 39 matching lines @@
                                                        String scriptClassName,
                                                        const char* source,
                                                        size_t size) {
   v8::Isolate* isolate = scriptState->isolate();
   v8::TryCatch block(isolate);
   String sourceString(source, size);
   String fileName = scriptClassName + ".js";
 
   v8::Local<v8::Context> context = scriptState->context();
   v8::Local<v8::Object> global = context->Global();
-  v8::Local<v8::Value> privateScriptController =
-      global->Get(context, v8String(isolate, "privateScriptController"))
-          .ToLocalChecked();
-  RELEASE_ASSERT(privateScriptController->IsUndefined() ||
-                 privateScriptController->IsObject());
-  if (privateScriptController->IsObject()) {
+  v8::Local<v8::String> key = v8String(isolate, "privateScriptController");
+
+  if (global->HasOwnProperty(context, key).ToChecked()) {
+    v8::Local<v8::Value> privateScriptController =
+        global->Get(context, key).ToLocalChecked();
+    CHECK(privateScriptController->IsObject());
     v8::Local<v8::Object> privateScriptControllerObject =
         privateScriptController.As<v8::Object>();
     v8::Local<v8::Value> importFunctionValue =
         privateScriptControllerObject->Get(context, v8String(isolate, "import"))
             .ToLocalChecked();
     if (importFunctionValue->IsUndefined()) {
       v8::Local<v8::Function> function;
       // This is a memory leak, FunctionTemplates are eternal.
       if (!v8::FunctionTemplate::New(isolate, importFunction)
                ->GetFunction(context)
@@ skipping 423 matching lines @@
     rethrowExceptionInPrivateScript(
         scriptState->isolate(), block, scriptStateInUserScript,
         ExceptionState::ExecutionContext, methodName, className);
     block.ReThrow();
     return v8::Local<v8::Value>();
   }
   return result;
 }
 
 }  // namespace blink