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

tools/clang/blink_gc_plugin/tests/class_requires_trace_method.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 CLASS_REQUIRES_TRACE_METHOD_H_
 #define CLASS_REQUIRES_TRACE_METHOD_H_
 
 #include "heap/stubs.h"
 
 namespace blink {
 
 class HeapObject;
 
 class PartObject {
     DISALLOW_ALLOCATION();
 private:
     Member<HeapObject> m_obj;
 };
 
 class HeapObject : public GarbageCollected<HeapObject> {
 private:
     PartObject m_part;
 };
 
+class Mixin : public GarbageCollectedMixin {
+public:
+  virtual void trace(Visitor*) override;
+  Member<Mixin> m_self;
+};
+
+class HeapObjectMixin : public GarbageCollected<HeapObjectMixin>, public Mixin {
+  USING_GARBAGE_COLLECTED_MIXIN(HeapObjectMixin);
+};
+
+class Mixin2 : public Mixin {
+public:
+  virtual void trace(Visitor*) override;
+};
+
+class HeapObjectMixin2
+    : public GarbageCollected<HeapObjectMixin2>, public Mixin2 {
+  USING_GARBAGE_COLLECTED_MIXIN(HeapObjectMixin2);
+};
+
+class Mixin3 : public Mixin {
+public:
+  virtual void trace(Visitor*) override;
+};
+
+class HeapObjectMixin3
+    : public GarbageCollected<HeapObjectMixin3>, public Mixin {
+  USING_GARBAGE_COLLECTED_MIXIN(HeapObjectMixin2);
+public:
+  virtual void trace(Visitor*) override;
+};
+
 }
 
 #endif