Blink GC plugin: Ensure that classes with virtual trace methods always have vtables for their left-…

tools/clang/blink_gc_plugin/tests/non_virtual_trace.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 NON_VIRTUAL_TRACE_H_
 #define NON_VIRTUAL_TRACE_H_
 
 #include "heap/stubs.h"
 
 namespace blink {
 
 class A : public GarbageCollected<A> {
 public:
     void trace(Visitor*);
 };
 
 class B : public A {
 };
 
 class C : public B {
 public:
     void trace(Visitor*); // Cannot override a non-virtual trace.
 };
 
+class D : public B {
+public:
+    virtual void trace(Visitor*); // Cannot override a non-virtual trace.
+};
+
 }
 
 #endif