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

tools/clang/blink_gc_plugin/CheckFieldsVisitor.cpp

 // Copyright 2015 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 "CheckFieldsVisitor.h"
 
 #include <cassert>
 
 #include "BlinkGCPluginOptions.h"
 #include "RecordInfo.h"
@@ skipping 89 matching lines @@
         invalid_fields_.push_back(std::make_pair(
             current_, kReferencePtrToGCManagedWarning));
       } else {
         invalid_fields_.push_back(std::make_pair(
             current_, kRawPtrToGCManagedWarning));
       }
     }
     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;
   }
 }
 
 void CheckFieldsVisitor::AtCollection(Collection* edge) {
   if (edge->on_heap() && Parent() && Parent()->IsOwnPtr())
     invalid_fields_.push_back(std::make_pair(current_, kOwnPtrToGCManaged));
 }
 
 bool CheckFieldsVisitor::IsWarning(Error error) {
   if (error == kRawPtrToGCManagedWarning)
@@ skipping 19 matching lines @@
       return kReferencePtrToGCManaged;
     else
       return kRawPtrToGCManaged;
   }
   if (ptr->IsRefPtr())
     return kRefPtrToGCManaged;
   if (ptr->IsOwnPtr())
     return kOwnPtrToGCManaged;
   assert(false && "Unknown smart pointer kind");
 }