Cosmetic improvements to marking of objects during GC

Change-Id: I79a7093c7086ea7f34252f097f18fe7c835053aa
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

src/qml/jsruntime/qv4engine_p.h

@@ -496,11 +496,12 @@ void Heap::Base::mark(QV4::ExecutionEngine *engine)
 
 inline void Value::mark(ExecutionEngine *e)
 {
-    if (!_val)
+    if (!isManaged())
         return;
-    Managed *m = as<Managed>();
-    if (m)
-        m->d()->mark(e);
+
+    Heap::Base *o = heapObject();
+    if (o)
+        o->mark(e);
 }
 
 

src/qml/memory/qv4mm.cpp

@@ -361,10 +361,10 @@ void MemoryManager::mark()
 
     engine->markObjects();
 
-    m_persistentValues->mark(engine);
-
     collectFromJSStack();
 
+    m_persistentValues->mark(engine);
+
     // Preserve QObject ownership rules within JavaScript: A parent with c++ ownership
     // keeps all of its children alive in JavaScript.