WIP (Introduce WTF::NonNullPtr<T>.)

Source/core/page/FocusController.h

Index: Source/core/page/FocusController.h
diff --git a/Source/core/page/FocusController.h b/Source/core/page/FocusController.h
index ff245c6fb849f0d83fbc85a9e34a098cc7c3b2a4..a0c5b343f1595c465699f9e498110eb5308039d3 100644
--- a/Source/core/page/FocusController.h
+++ b/Source/core/page/FocusController.h
@@ -29,6 +29,7 @@
 #include "core/page/FocusDirection.h"
 #include "core/platform/graphics/LayoutRect.h"
 #include "wtf/Forward.h"
+#include "wtf/NonNullPtr.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/RefPtr.h"
 
@@ -57,8 +58,8 @@ public:
     static FocusNavigationScope ownedByIFrame(HTMLFrameOwnerElement*);
 
 private:
-    explicit FocusNavigationScope(TreeScope*);
-    TreeScope* m_rootTreeScope;
+    explicit FocusNavigationScope(NonNullPtr<TreeScope>);
+    NonNullPtr<TreeScope> m_rootTreeScope;
 };
 
 class FocusController {