Blink GC plugin: require a trace method if a class derives multiple bases that need tracing.

tools/clang/blink_gc_plugin/tests/class_multiple_trace_bases.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 GARBAGE_COLLECTED_MIXIN_H_
-#define GARBAGE_COLLECTED_MIXIN_H_
+#ifndef CLASS_MULTIPLE_TRACE_BASES_H_
+#define CLASS_MULTIPLE_TRACE_BASES_H_
 
 #include "heap/stubs.h"
 
 namespace WebCore {
 
+class Base : public GarbageCollected<Base> {
+public:
+    virtual void trace(Visitor*);
+};
+
 class Mixin : public GarbageCollectedMixin {
 public:
     void trace(Visitor*);
-private:
-    Member<Mixin> m_self;
 };
 
-class HeapObject : public GarbageCollected<HeapObject>, public Mixin {
+class Derived : public Base, public Mixin {
     USING_GARBAGE_COLLECTED_MIXIN(HeapObject);
-public:
-    void trace(Visitor*);
-private:
-    Member<Mixin> m_mix;
+    // Requires trace method.
 };
 
 }
 
 #endif