Use return value of GetPropertyAttributes to make compiler happy.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index c195a90271a18b8a3d33f68d5ffbaa48469e6a2d..a3ebfd34bc9029ab2293868fcd8daf9829c8f151 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -5251,7 +5251,8 @@ RUNTIME_FUNCTION(Runtime_AddNamedProperty) {
   uint32_t index = 0;
   ASSERT(!key->ToArrayIndex(&index));
   LookupIterator it(object, key, LookupIterator::CHECK_OWN_REAL);
-  JSReceiver::GetPropertyAttributes(&it);
+  Maybe<PropertyAttributes> maybe = JSReceiver::GetPropertyAttributes(&it);
+  ASSERT(maybe.has_value);
   RUNTIME_ASSERT(!it.IsFound());
 #endif
 
@@ -5282,7 +5283,8 @@ RUNTIME_FUNCTION(Runtime_AddPropertyForTemplate) {
   if (key->IsName()) {
     LookupIterator it(object, Handle<Name>::cast(key),
                       LookupIterator::CHECK_OWN_REAL);
-    JSReceiver::GetPropertyAttributes(&it);
+    Maybe<PropertyAttributes> maybe = JSReceiver::GetPropertyAttributes(&it);
+    ASSERT(maybe.has_value);
     duplicate = it.IsFound();
   } else {
     uint32_t index = 0;