[runtime] JSObject::GetKeys never returns the EnumCache, so we don't need to copy

BUG=148757
LOG=n

Review URL: https://codereview.chromium.org/1768443003

Cr-Commit-Position: refs/heads/master@{#34495}

src/api.cc

@@ -3791,11 +3791,10 @@ MaybeLocal<Array> v8::Object::GetOwnPropertyNames(Local<Context> context) {
       !i::JSReceiver::GetKeys(self, i::OWN_ONLY, i::ENUMERABLE_STRINGS)
            .ToHandle(&value);
   RETURN_ON_FAILED_EXECUTION(Array);
-  // Because we use caching to speed up enumeration it is important
-  // to never change the result of the basic enumeration function so
-  // we clone the result.
-  auto elms = isolate->factory()->CopyFixedArray(value);
-  auto result = isolate->factory()->NewJSArrayWithElements(elms);
+  DCHECK(self->map()->EnumLength() == i::kInvalidEnumCacheSentinel ||
+         self->map()->EnumLength() == 0 ||
+         self->map()->instance_descriptors()->GetEnumCache() != *value);
+  auto result = isolate->factory()->NewJSArrayWithElements(value);
   RETURN_ESCAPED(Utils::ToLocal(result));
 }