Address two review comments from 9531001.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
diff --git a/runtime/vm/gc_marker.cc b/runtime/vm/gc_marker.cc
index f2c2a07bc55f32fa6f2f79dcf62bd7908750ffc6..e7648f72947d65810ab7fe662fa3e0b481ebfe12 100644
--- a/runtime/vm/gc_marker.cc
+++ b/runtime/vm/gc_marker.cc
@@ -240,10 +240,11 @@ void GCMarker::IterateWeakReferences(Isolate* isolate,
                                      MarkingVisitor* visitor) {
   ApiState* state = isolate->api_state();
   ASSERT(state != NULL);
-  for (;;) {
+  while (true) {
     WeakReference* queue = state->delayed_weak_references();
     if (queue == NULL) {
-      break;
+      // The delay queue is empty therefore no clean-up is required.
+      return;
     }
     state->set_delayed_weak_references(NULL);
     while (queue != NULL) {