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

tools/clang/blink_gc_plugin/CheckFieldsVisitor.cpp

Index: tools/clang/blink_gc_plugin/CheckFieldsVisitor.cpp
diff --git a/tools/clang/blink_gc_plugin/CheckFieldsVisitor.cpp b/tools/clang/blink_gc_plugin/CheckFieldsVisitor.cpp
index 0ed953fccf92e57197fcb7cc54a99bff36b8a675..558637c9c190b5489e0ddccf37b0c62f921568af 100644
--- a/tools/clang/blink_gc_plugin/CheckFieldsVisitor.cpp
+++ b/tools/clang/blink_gc_plugin/CheckFieldsVisitor.cpp
@@ -107,7 +107,13 @@ void CheckFieldsVisitor::AtValue(Value* edge) {
     return;
   }
 
-  if (Parent()->IsRawPtr() || Parent()->IsRefPtr() || Parent()->IsOwnPtr()) {
+  if (Parent()->IsRawPtr() || Parent()->IsOwnPtr()) {
+    invalid_fields_.push_back(std::make_pair(
+        current_, InvalidSmartPtr(Parent())));
+    return;
+  }
+
+  if (Parent()->IsRefPtr() && !edge->value()->IsGCRefCounted()) {
     invalid_fields_.push_back(std::make_pair(
         current_, InvalidSmartPtr(Parent())));
     return;