blink_gc_plugin: detect singletons with embedded ScriptWrappables.

tools/clang/blink_gc_plugin/tests/heap/stubs.h

Index: tools/clang/blink_gc_plugin/tests/heap/stubs.h
diff --git a/tools/clang/blink_gc_plugin/tests/heap/stubs.h b/tools/clang/blink_gc_plugin/tests/heap/stubs.h
index 4f172734c83110e3dca98b3fd840ff9f604d9ee1..0e3d961baeb94f143908a66cd0375d924d5e6a01 100644
--- a/tools/clang/blink_gc_plugin/tests/heap/stubs.h
+++ b/tools/clang/blink_gc_plugin/tests/heap/stubs.h
@@ -188,6 +188,9 @@ using namespace WTF;
 #define GC_PLUGIN_IGNORE(bug)                           \
     __attribute__((annotate("blink_gc_plugin_ignore")))
 
+#define BLINK_STATIC_SINGLETON() \
+  __attribute__((annotate("blink_gc_singleton_type")))
+
 #define USING_GARBAGE_COLLECTED_MIXIN(type)                     \
 public:                                                         \
     virtual void AdjustAndMark(Visitor*) const override { }     \
@@ -219,9 +222,10 @@ public:
 
 template<typename T> class Persistent {
 public:
-    operator T*() const { return 0; }
-    T* operator->() { return 0; }
-    bool operator!() const { return false; }
+ explicit Persistent(T*) {}
+ operator T*() const { return 0; }
+ T* operator->() { return 0; }
+ bool operator!() const { return false; }
 };
 
 template<typename T> class WeakPersistent {
@@ -297,7 +301,9 @@ struct TraceIfNeeded {
     static void Trace(Visitor*, T*);
 };
 
-}
+class ScriptWrappable {};
+
+}  // namespace blink
 
 namespace WTF {