blink_gc_plugin: warn of unused trace methods to stack allocated classes.

tools/clang/blink_gc_plugin/tests/stack_allocated.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 STACK_ALLOCATED_H_
 #define STACK_ALLOCATED_H_
 
 #include "heap/stubs.h"
 
 namespace blink {
 
 class HeapObject;
 
 class PartObject {
     DISALLOW_NEW();
 private:
     Member<HeapObject> m_obj; // Needs tracing.
 };
 
 class StackObject {
     STACK_ALLOCATED();
+
+    // Redundant trace() method, warning/error expected.
+    void Trace(Visitor* visitor) { visitor->Trace(m_obj); }
+
 private:
     Member<HeapObject> m_obj; // Does not need tracing.
 };
 
 class HeapObject : public GarbageCollected<HeapObject> {
 public:
     void Trace(Visitor*);
 private:
     StackObject m_part; // Cannot embed a stack allocated object.
 };
 
 // Cannot derive from both heap- and stack-allocated objects.
 class DerivedHeapObject : public HeapObject, public StackObject {
 };
 
 // Cannot be stack-allocated and derive from a heap-allocated object.
 class DerivedHeapObject2 : public HeapObject {
   STACK_ALLOCATED();
 };
 
 // STACK_ALLOCATED is inherited.
 class DerivedStackObject : public StackObject {
 private:
     StackObject m_anotherPart; // Also fine.
 };
 
 }
 
 #endif