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

tools/clang/blink_gc_plugin/tests/member_in_offheap_class.h

 // 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.
 
 #ifndef MEMBER_IN_OFFHEAP_CLASS_H_
 #define MEMBER_IN_OFFHEAP_CLASS_H_
 
 #include "heap/stubs.h"
 
 namespace blink {
 
 class HeapObject : public GarbageCollected<HeapObject> { };
 
 class OffHeapObject {
 public:
     void Trace(Visitor*);
 private:
     Member<HeapObject> m_obj; // Must not contain Member.
+    WeakMember<HeapObject> m_weak;  // Must not contain WeakMember.
     Persistent<HeapVector<Member<HeapObject> > > m_objs; // OK
 };
 
 class StackObject {
     STACK_ALLOCATED();
 private:
     Member<HeapObject> m_obj; // OK
     Member<OffHeapObject> m_memberOff; // NOT OK
     HeapVector<Member<OffHeapObject>> m_heapVectorMemberOff; // NOT OK
 };
@@ skipping 10 matching lines @@
     ALLOW_ONLY_INLINE_ALLOCATION();
 public:
     void Trace(Visitor*);
 private:
     Member<HeapObject> m_obj; // OK
 };
 
 }
 
 #endif