GC plugin: RefPtr<T> is legal if T is RefCountedGC-derived.

tools/clang/blink_gc_plugin/Config.h

Index: tools/clang/blink_gc_plugin/Config.h
diff --git a/tools/clang/blink_gc_plugin/Config.h b/tools/clang/blink_gc_plugin/Config.h
index c62de053306072539003ef93baead1c3dd3a13db..7ca8ea4c0f176fce6a46ea9eccc2005c49b3826b 100644
--- a/tools/clang/blink_gc_plugin/Config.h
+++ b/tools/clang/blink_gc_plugin/Config.h
@@ -138,6 +138,11 @@ class Config {
            name == "ThreadSafeRefCountedGarbageCollected";
   }
 
+  static bool IsGCRefCountedBase(const std::string& name) {
+    return name == "RefCountedGarbageCollected" ||
+           name == "ThreadSafeRefCountedGarbageCollected";
+  }
+
   static bool IsGCBase(const std::string& name) {
     return name == "GarbageCollected" ||
            IsGCFinalizedBase(name) ||