Oilpan: Remove most WillBe types from the code base

third_party/WebKit/Source/core/dom/DocumentVisibilityObserver.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 DocumentVisibilityObserver_h
 #define DocumentVisibilityObserver_h
 
 #include "core/CoreExport.h"
 #include "core/page/PageVisibilityState.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class Document;
 
-class CORE_EXPORT DocumentVisibilityObserver : public WillBeGarbageCollectedMixin {
+class CORE_EXPORT DocumentVisibilityObserver : public GarbageCollectedMixin {
     WTF_MAKE_NONCOPYABLE(DocumentVisibilityObserver);
 public:
     explicit DocumentVisibilityObserver(Document&);
     virtual ~DocumentVisibilityObserver();
 
     virtual void didChangeVisibilityState(PageVisibilityState) = 0;
     virtual void willDetachDocument() = 0;
 
     // Classes that inherit Node and DocumentVisibilityObserver must have a
     // virtual override of Node::didMoveToNewDocument that calls
     // DocumentVisibilityObserver::setDocument
     void setObservedDocument(Document&);
 
 protected:
     DECLARE_VIRTUAL_TRACE();
 
 private:
     void registerObserver(Document&);
     void unregisterObserver();
 
-    RawPtrWillBeMember<Document> m_document;
+    Member<Document> m_document;
 };
 
 } // namespace blink
 
 #endif // DocumentVisibilityObserver_h