blink_gc_plugin should require tracing for classes that have a trace method

tools/clang/blink_gc_plugin/tests/base_with_trace_requires_tracing.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 BASE_WITH_TRACE_REQUIRES_TRACING_H_
+#define BASE_WITH_TRACE_REQUIRES_TRACING_H_
+
+#include "heap/stubs.h"
+
+namespace blink {
+
+class A : public GarbageCollected<A> {
+public:
+    virtual void trace(Visitor*);
+};
+
+class B : public A {
+    void trace(Visitor*);
+private:
+    // Ignoring all fields will mean this does not requiring tracing. But if a
+    // trace method is defined explicitly we should require tracing.
+    GC_PLUGIN_IGNORE("http://crbug.com/12345")
+    Member<A> m_a;
+};
+
+class C : public B {
+public:
+    void trace(Visitor*);
+private:
+    Member<A> m_a;
+};
+
+}
+
+#endif