Oilpan: Remove guardRef and guardDeref from TreeScope.

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index 659a2cc4889d1c5c882952be6401ba919c1537d1..3dc05123461b81e54db3e2259e6734789113670f 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -213,7 +213,7 @@ typedef unsigned char DocumentClassFlags;
 
 class Document;
 
-class DocumentVisibilityObserver {
+class DocumentVisibilityObserver : public WillBeGarbageCollectedMixin {
 public:
     DocumentVisibilityObserver(Document&);
     virtual ~DocumentVisibilityObserver();
@@ -857,7 +857,7 @@ public:
     void cancelFocusAppearanceUpdate();
 
     // Extension for manipulating canvas drawing contexts for use in CSS
-    void getCSSCanvasContext(const String& type, const String& name, int width, int height, bool&, RefPtr<CanvasRenderingContext2D>&, bool&, RefPtr<WebGLRenderingContext>&);
+    void getCSSCanvasContext(const String& type, const String& name, int width, int height, bool&, RefPtrWillBeRawPtr<CanvasRenderingContext2D>&, bool&, RefPtrWillBeRawPtr<WebGLRenderingContext>&);
     HTMLCanvasElement& getCSSCanvasElement(const String& name);
 
     bool isDNSPrefetchEnabled() const { return m_isDNSPrefetchEnabled; }
@@ -1101,6 +1101,8 @@ private:
 
     void detachParser();
 
+    void clearWeakMembers(Visitor*);
+
     virtual bool isDocument() const OVERRIDE FINAL { return true; }
 
     virtual void childrenChanged(bool changedByParser = false, Node* beforeChange = 0, Node* afterChange = 0, int childCountDelta = 0) OVERRIDE;
@@ -1370,7 +1372,7 @@ private:
 
     bool m_hasViewportUnits;
 
-    HashSet<DocumentVisibilityObserver*> m_visibilityObservers;
+    WillBeHeapHashSet<RawPtrWillBeWeakMember<DocumentVisibilityObserver> > m_visibilityObservers;
 };
 
 inline void Document::notifyRemovePendingSheetIfNeeded()