Object.preventExtensions should return object even if object is proxy.

According to spec https://tc39.es/ecma262/#sec-object.preventextensions, the commit 8e0ef9b9 is missing the last step when object is proxy, it needs to return the object.
var proxy = new Proxy({}, {});
var object = Object.preventExtensions(proxy);
proxy === object; // should be true

Also, add mjsunit test.

Bug: v8:6664
Change-Id: Ic3688519539f8903ee0bc7e885905a86d195a4db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1668443
Commit-Queue: Z Nguyen-Huu <duongn@microsoft.com>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62393}

src/builtins/object.tq

@@ -34,7 +34,8 @@ namespace object {
     const objectJSProxy = Cast<JSProxy>(objectJSReceiver)
         otherwise return runtime::JSReceiverPreventExtensionsThrow(
         objectJSReceiver);
-    return proxy::ProxyPreventExtensions(objectJSProxy, True);
+    const _status: Object = proxy::ProxyPreventExtensions(objectJSProxy, True);
+    return objectJSReceiver;
   }
 
   transitioning macro

test/mjsunit/es6/proxies-prevent-extensions.js

@@ -105,7 +105,7 @@
 
   assertTrue(Object.isExtensible(target));
   assertTrue(Object.isExtensible(proxy));
-  Object.preventExtensions(proxy);
+  assertSame(proxy, Object.preventExtensions(proxy));
   assertFalse(Object.isExtensible(target));
   assertFalse(Object.isExtensible(proxy));
 })();
@@ -119,7 +119,7 @@
 
   assertTrue(Object.isExtensible(target));
   assertTrue(Object.isExtensible(proxy));
-  Object.preventExtensions(proxy);
+  assertSame(proxy, Object.preventExtensions(proxy));
   assertFalse(Object.isExtensible(target));
   assertFalse(Object.isExtensible(proxy));
 })();
@@ -145,7 +145,7 @@
   assertTrue(Object.isExtensible(proxy));
 
   // Trap returns trueish but target is not extensible.
-  Object.preventExtensions(target);
+  assertSame(target, Object.preventExtensions(target));
   assertThrows(() => { Object.isExtensible(proxy) }, TypeError);
 })();
 
@@ -158,7 +158,7 @@
   var proxy = new Proxy(target, handler);
 
   assertThrows(() => { Object.isExtensible(proxy) }, TypeError);
-  Object.preventExtensions(target);
+  assertSame(target, Object.preventExtensions(target));
   assertFalse(Object.isExtensible(proxy));
 })();
 
@@ -171,7 +171,7 @@
   var proxy = new Proxy(target, handler);
 
   assertFalse(Reflect.preventExtensions(proxy));
-  Object.preventExtensions(target);
+  assertSame(target, Object.preventExtensions(target));
   assertFalse(Reflect.preventExtensions(proxy));
   assertThrows(() => { Object.preventExtensions(proxy) }, TypeError);
 })();
@@ -186,7 +186,7 @@
   assertThrows(() => { Object.preventExtensions(proxy) }, TypeError);
 
   // Trap returns trueish and target is not extensible.
-  Object.preventExtensions(target);
+  assertSame(target, Object.preventExtensions(target));
   assertTrue(Reflect.preventExtensions(proxy));
 })();
 
@@ -194,7 +194,7 @@
 (function () {
   // Target is proxy
   var object = {};
-  Object.preventExtensions(object);
+  assertSame(object, Object.preventExtensions(object));
   var target = new Proxy(object, {});
   var proxy = new Proxy(target, {});
   assertFalse(Object.isExtensible(object));