GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/tests/persistent_field_in_gc_managed_class.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 PERSISTENT_FIELD_IN_GC_MANAGED_CLASS_H_
 #define PERSISTENT_FIELD_IN_GC_MANAGED_CLASS_H_
 
 #include "heap/stubs.h"
 
 namespace blink {
 
 class HeapObject;
 
 class PartObject {
     DISALLOW_ALLOCATION();
 private:
     Persistent<HeapObject> m_obj;
 };
 
 class HeapObject : public GarbageCollected<HeapObject> {
 public:
     void trace(Visitor*);
 private:
     PartObject m_part;
     HeapVector<PartObject> m_parts;
     PersistentHeapVector<Member<HeapObject> > m_objs;
+    WeakPersistent<HeapObject> m_weakPersistent;
 };
 
 }
 
 #endif