GC plugin: improve error reporting when tracing illegal fields.

tools/clang/blink_gc_plugin/tests/fields_illegal_tracing.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 "fields_illegal_tracing.h"
+
+namespace blink {
+
+void PartObject::trace(Visitor* visitor) {
+    visitor->trace(m_obj1);
+    visitor->trace(m_obj2);
+    visitor->trace(m_obj3);
+}
+
+void HeapObject::trace(Visitor* visitor) {
+    visitor->trace(m_obj1);
+    visitor->trace(m_obj2);
+    visitor->trace(m_obj3);
+}
+
+}