GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/tests/persistent_no_trace.cpp

 // Copyright 2016 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 "platform/text/Hyphenation.h"
+#include "persistent_no_trace.h"
 
 namespace blink {
 
-PassRefPtr<Hyphenation> Hyphenation::platformGetHyphenation(const AtomicString&)
-{
-    return nullptr;
+void HeapObject::trace(Visitor* visitor) {
+    visitor->trace(m_crossThreadPersistent);
+    visitor->trace(m_crossThreadWeakPersistent);
 }
 
-} // namespace blink
+}