GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/tests/fields_illegal_tracing.h

+// Copyright 2016 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_ILLEGAL_TRACING_H_
+#define FIELDS_ILLEGAL_TRACING_H_
+
+#include "heap/stubs.h"
+
+namespace blink {
+
+class HeapObject;
+class PartObject;
+
+class PartObject {
+    DISALLOW_ALLOCATION();
+public:
+    void trace(Visitor*);
+private:
+    OwnPtr<HeapObject> m_obj1;
+    std::unique_ptr<HeapObject> m_obj2;
+    RefPtr<HeapObject> m_obj3;
+};
+
+class HeapObject : public GarbageCollectedFinalized<HeapObject> {
+public:
+    void trace(Visitor*);
+private:
+    PartObject m_part;
+    OwnPtr<HeapObject> m_obj1;
+    RefPtr<HeapObject> m_obj2;
+    std::unique_ptr<HeapObject> m_obj3;
+};
+
+}
+
+#endif