blink_gc_plugin should require tracing for classes that have a trace method

tools/clang/blink_gc_plugin/tests/base_with_trace_requires_tracing.cpp

+// 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.
+
+#include "base_with_trace_requires_tracing.h"
+
+namespace blink {
+
+void A::trace(Visitor* visitor) { }
+
+void B::trace(Visitor* visitor) {
+  visitor->trace(m_a);
+  A::trace(visitor);
+}
+
+void C::trace(Visitor* visitor) {
+  visitor->trace(m_a);
+  A::trace(visitor); // This should be B::trace()
+}
+
+}