Oilpan: Remove guardRef and guardDeref from TreeScope.

Source/core/page/Page.h

Index: Source/core/page/Page.h
diff --git a/Source/core/page/Page.h b/Source/core/page/Page.h
index f74630236d080931219cee940960c534e1482841..58196f59ae8581bd24cc3ffc856e98e9e191e7ee 100644
--- a/Source/core/page/Page.h
+++ b/Source/core/page/Page.h
@@ -31,6 +31,7 @@
 #include "platform/Supplementable.h"
 #include "platform/geometry/LayoutRect.h"
 #include "platform/geometry/Region.h"
+#include "platform/heap/Handle.h"
 #include "wtf/Forward.h"
 #include "wtf/HashSet.h"
 #include "wtf/Noncopyable.h"
@@ -206,7 +207,7 @@ public:
 
     double timerAlignmentInterval() const;
 
-    class MultisamplingChangedObserver {
+    class MultisamplingChangedObserver : public WillBeGarbageCollectedMixin {
     public:
         virtual void multisamplingChanged(bool) = 0;
     };
@@ -220,7 +221,7 @@ public:
     PassOwnPtr<LifecycleNotifier<Page> > createLifecycleNotifier();
 
     virtual void trace(Visitor*);
-
+    void clearWeakMembers(Visitor*);
     void willBeDestroyed();
 
 protected:
@@ -289,7 +290,7 @@ private:
     bool m_isPainting;
 #endif
 
-    HashSet<MultisamplingChangedObserver*> m_multisamplingChangedObservers;
+    WillBeHeapHashSet<RawPtrWillBeWeakMember<MultisamplingChangedObserver> > m_multisamplingChangedObservers;
 
     // A pointer to all the interfaces provided to in-process Frames for this Page.
     // FIXME: Most of the members of Page should move onto FrameHost.