Bug 1449033 - Set new group unknown flag on placeholder prototypes where necessary r=jandem

js/src/gc/GC.cpp

@@ -8042,10 +8042,11 @@ GCRuntime::mergeCompartments(JSCompartment* source, JSCompartment* target)
             if (GlobalObject::isOffThreadPrototypePlaceholder(obj)) {
                 JSObject* targetProto = global->getPrototypeForOffThreadPlaceholder(obj);
                 MOZ_ASSERT(targetProto->isDelegate());
+                MOZ_ASSERT_IF(targetProto->staticPrototypeIsImmutable(),
+                              obj->staticPrototypeIsImmutable());
+                MOZ_ASSERT_IF(targetProto->isNewGroupUnknown(),
+                              obj->isNewGroupUnknown());
                 group->setProtoUnchecked(TaggedProto(targetProto));
-                AutoSweepObjectGroup sweep(group);
-                if (targetProto->isNewGroupUnknown() && !group->unknownProperties(sweep))
-                    group->markUnknown(sweep, cx);
             }
         }
 

js/src/vm/GlobalObject.cpp

@@ -343,6 +343,12 @@ GlobalObject::resolveOffThreadConstructor(JSContext* cx,
         return false;
     }
 
+    if ((key == JSProto_Object || key == JSProto_Function || key == JSProto_Array) &&
+        !JSObject::setNewGroupUnknown(cx, placeholder->getClass(), placeholder))
+    {
+        return false;
+    }
+
     global->setPrototype(key, ObjectValue(*placeholder));
     global->setConstructor(key, MagicValue(JS_OFF_THREAD_CONSTRUCTOR));
     return true;