Blink GC plugin: Gracefully handle cyclic part objects.

tools/clang/blink_gc_plugin/tests/fields_require_tracing.h

 // 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.
 
 #ifndef FIELDS_REQUIRE_TRACING_H_
 #define FIELDS_REQUIRE_TRACING_H_
 
 #include "heap/stubs.h"
 
 namespace blink {
 
 class HeapObject;
+class PartObject;
+
+class PartBObject {
+    DISALLOW_ALLOCATION();
+public:
+    void trace(Visitor*);
+private:
+    HeapHashSet<PartBObject> m_set;
+    HeapVector<PartBObject> m_vector;
+};
 
 class PartObject {
     DISALLOW_ALLOCATION();
 public:
     void trace(Visitor*);
 private:
     Member<HeapObject> m_obj1;
     Member<HeapObject> m_obj2;
     Member<HeapObject> m_obj3;
+
+    HeapVector<PartBObject> m_parts;
 };
 
 class HeapObject : public GarbageCollected<HeapObject> {
 public:
     void trace(Visitor*);
 private:
     PartObject m_part;
     Member<HeapObject> m_obj;
 };
 
 }
 
 #endif