Ensure that Heap.h (not Visitor.h) is #included from Oilpan user code

Source/web/PluginPlaceholderImpl.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 PluginPlaceholderImpl_h
 #define PluginPlaceholderImpl_h
 
 #include "core/plugins/PluginPlaceholder.h"
 #include "platform/heap/Handle.h"
-#include "platform/heap/Visitor.h"
 #include "wtf/Assertions.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
 
 class Document;
 class PluginPlaceholderElement;
 class WebPluginPlaceholder;
 
@@ skipping 29 matching lines @@
     // Update the placeholder element with fresh content.
     void update();
 
     OwnPtr<WebPluginPlaceholder> m_webPluginPlaceholder;
     RefPtrWillBeMember<PluginPlaceholderElement> m_placeholderElement;
 };
 
 } // namespace blink
 
 #endif // PluginPlaceholderImpl_h