blink_gc_plugin: disallow WeakMember<> fields in off-heap objects.

tools/clang/blink_gc_plugin/tests/member_in_offheap_class.cpp

 // Copyright 2014 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 "member_in_offheap_class.h"
 
 namespace blink {
 
 void OffHeapObject::Trace(Visitor* visitor)
 {
     visitor->Trace(m_obj);
+    visitor->Trace(m_weak);
 }
 
 void PartObject::Trace(Visitor* visitor)
 {
     visitor->Trace(m_obj);
 }
 
 void InlineObject::Trace(Visitor* visitor)
 {
     visitor->Trace(m_obj);
 }
 
 }