Update Blink GC plugin to reflect that Oilpan is now always enabled.

tools/clang/blink_gc_plugin/tests/raw_ptr_to_gc_managed_class_error.h

 // 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.
 
 #ifndef RAW_PTR_TO_GC_MANAGED_CLASS_ERROR_H_
 #define RAW_PTR_TO_GC_MANAGED_CLASS_ERROR_H_
 
 #include "heap/stubs.h"
 
 namespace blink {
 
 class HeapObject;
 
 class PartObject {
     DISALLOW_ALLOCATION();
 private:
     PartObject();
 
-    RawPtr<HeapObject> m_obj;
     HeapObject* m_rawObj;
     HeapObject& m_refObj;
 };
 
 class HeapObject : public GarbageCollected<HeapObject> {
 public:
     void trace(Visitor*);
 private:
     PartObject m_part;
     HeapVector<HeapObject*> m_objs;
 };
 
 }
 
 #endif