Blink GC plugin: Gracefully handle cyclic part objects.

tools/clang/blink_gc_plugin/tests/fields_require_tracing.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "fields_require_tracing.h"
 
 namespace blink {
 
 void PartObject::trace(Visitor* visitor) {
     m_obj1->trace(visitor); // Don't allow direct tracing.
     visitor->trace(m_obj2);
     // Missing visitor->trace(m_obj3);
+    visitor->trace(m_parts);
+}
+
+void PartBObject::trace(Visitor* visitor) {
+  // Missing visitor->trace(m_set);
+  visitor->trace(m_vector);
 }
 
 void HeapObject::trace(Visitor* visitor) {
     // Missing visitor->trace(m_part);
     visitor->trace(m_obj);
 }
 
 }