[runtime] Fix Object.create(null) initialization order

A GC might cause the just created dictionary object to have an invalid backing
store, which breaks heap verification.

BUG=chromium:659088

Review-Url: https://codereview.chromium.org/2452653002
Cr-Commit-Position: refs/heads/master@{#40574}

src/runtime/runtime-object.cc

@@ -247,13 +247,18 @@ RUNTIME_FUNCTION(Runtime_ObjectCreate) {
     }
   }
 
+  bool is_dictionary_map = map->is_dictionary_map();
+  Handle<FixedArray> object_properties;
+  if (is_dictionary_map) {
+    // Allocate the actual properties dictionay up front to avoid invalid object
+    // state.
+    object_properties =
+        NameDictionary::New(isolate, NameDictionary::kInitialCapacity);
+  }
   // Actually allocate the object.
   Handle<JSObject> object = isolate->factory()->NewJSObjectFromMap(map);
-
-  if (map->is_dictionary_map()) {
-    Handle<NameDictionary> properties =
-        NameDictionary::New(isolate, NameDictionary::kInitialCapacity);
-    object->set_properties(*properties);
+  if (is_dictionary_map) {
+    object->set_properties(*object_properties);
   }
 
   // Define the properties if properties was specified and is not undefined.