blink_gc_plugin: detect singletons with embedded ScriptWrappables.

tools/clang/blink_gc_plugin/tests/static_singleton.h

Index: tools/clang/blink_gc_plugin/tests/static_singleton.h
diff --git a/tools/clang/blink_gc_plugin/tests/static_singleton.h b/tools/clang/blink_gc_plugin/tests/static_singleton.h
new file mode 100644
index 0000000000000000000000000000000000000000..9534f29b65e80b944d9116fb76dc7c8d34f99a80
--- /dev/null
+++ b/tools/clang/blink_gc_plugin/tests/static_singleton.h
@@ -0,0 +1,39 @@
+// Copyright 2017 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 STATIC_SINGLETON_H_
+#define STATIC_SINGLETON_H_
+
+#include "heap/stubs.h"
+
+namespace blink {
+
+class AScriptWrappable : public GarbageCollected<AScriptWrappable>,
+                         public ScriptWrappable {
+  void Trace(Visitor* visitor) { visitor->Trace(a_); }
+
+ private:
+  Member<AScriptWrappable> a_;
+};
+
+class BScriptWrappable : public GarbageCollected<AScriptWrappable> {
+  void Trace(Visitor* visitor) { visitor->Trace(a_); }
+
+ private:
+  Member<AScriptWrappable> a_;
+};
+
+class HeapObject : public GarbageCollected<HeapObject> {
+ public:
+  static HeapObject& instance();
+
+  void Trace(Visitor* visitor) { visitor->Trace(b_); }
+
+ private:
+  Member<BScriptWrappable> b_;
+};
+
+}  // namespace blink
+
+#endif  // STATIC_SINGLETON_H_