Fix GC mole warning.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 16164342a9a11ece230d99afc1842009c1f2d0a0..32ecf52e879b9421e36c32a06f9da4dea4c45a88 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -1714,7 +1714,8 @@ RUNTIME_FUNCTION(Runtime_GetWeakMapEntries) {
       Handle<Object> key(table->KeyAt(i), isolate);
       if (table->IsKey(*key)) {
         entries->set(number_of_non_hole_elements++, *key);
-        entries->set(number_of_non_hole_elements++, table->Lookup(key));
+        Object* value = table->Lookup(key);
+        entries->set(number_of_non_hole_elements++, value);
       }
     }
     DCHECK_EQ(table->NumberOfElements() * 2, number_of_non_hole_elements);