BlinkGCPlugin: require GC mixin instances to declare local trace().

tools/clang/blink_gc_plugin/tests/garbage_collected_mixin.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_
 
 #include "heap/stubs.h"
 
 namespace blink {
 
 class Mixin : public GarbageCollectedMixin {
 public:
-    void trace(Visitor*);
+    virtual void trace(Visitor*) override;
 private:
     Member<Mixin> m_self;
 };
 
 class HeapObject : public GarbageCollected<HeapObject>, public Mixin {
     USING_GARBAGE_COLLECTED_MIXIN(HeapObject);
 public:
-    void trace(Visitor*);
+    virtual void trace(Visitor*) override;
 private:
     Member<Mixin> m_mix;
 };
 
 }
 
 #endif