Move the Widget hierarchy to the Oilpan heap.

Source/platform/exported/WebScrollbarImpl.h

Index: Source/platform/exported/WebScrollbarImpl.h
diff --git a/Source/platform/exported/WebScrollbarImpl.h b/Source/platform/exported/WebScrollbarImpl.h
index 188f9ddb209d7c23967b2696a44f057d145cf0f4..e8bc43160598f15c5c3900d2cf030d035614eacb 100644
--- a/Source/platform/exported/WebScrollbarImpl.h
+++ b/Source/platform/exported/WebScrollbarImpl.h
@@ -26,14 +26,18 @@
 #define WebScrollbarImpl_h
 
 #include "platform/PlatformExport.h"
+#include "platform/heap/Handle.h"
 #include "public/platform/WebScrollbar.h"
 
 namespace blink {
 
 class Scrollbar;
-class PLATFORM_EXPORT WebScrollbarImpl : public WebScrollbar {
+class PLATFORM_EXPORT WebScrollbarImpl final : public WebScrollbar {
 public:
-    explicit WebScrollbarImpl(Scrollbar*);
+    static WebScrollbarImpl* create(Scrollbar* scrollbar)
+    {
+        return new WebScrollbarImpl(scrollbar);
+    }
 
     // Implement WebScrollbar methods
     virtual bool isOverlay() const override;
@@ -57,7 +61,9 @@ public:
     virtual void setIsAlphaLocked(bool) override;
 
 private:
-    RefPtr<Scrollbar> m_scrollbar;
+    explicit WebScrollbarImpl(Scrollbar*);
+
+    RefPtrWillBePersistent<Scrollbar> m_scrollbar;
 };
 
 } // namespace blink