Oilpan: have Navigator plugin objects on the heap by default.

Source/modules/plugins/NavigatorPlugins.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 NavigatorPlugins_h
 #define NavigatorPlugins_h
 
 #include "core/frame/DOMWindowProperty.h"
 #include "platform/Supplementable.h"
 
@@ skipping 20 matching lines @@
 
 private:
     explicit NavigatorPlugins(Navigator&);
 
     static const char* supplementName();
 
     DOMPluginArray* plugins(LocalFrame*) const;
     DOMMimeTypeArray* mimeTypes(LocalFrame*) const;
     bool javaEnabled(LocalFrame*) const;
 
-    mutable RefPtrWillBeMember<DOMPluginArray> m_plugins;
-    mutable RefPtrWillBeMember<DOMMimeTypeArray> m_mimeTypes;
+    mutable Member<DOMPluginArray> m_plugins;
+    mutable Member<DOMMimeTypeArray> m_mimeTypes;
 };
 
 } // namespace blink
 
 #endif // NavigatorPlugins_h