Make VisiblePosition::init to not use being constructed object

Source/core/editing/PositionWithAffinity.h

 // Copyright 2014 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 PositionWithAffinity_h
 #define PositionWithAffinity_h
 
 #include "core/CoreExport.h"
 #include "core/dom/Position.h"
 #include "core/editing/TextAffinity.h"
 
 namespace blink {
 
 template <typename Position>
 class CORE_TEMPLATE_CLASS_EXPORT PositionWithAffinityTemplate {
     DISALLOW_ALLOCATION();
 public:
     typedef Position PositionType;
 
     PositionWithAffinityTemplate(const PositionType&, EAffinity = DOWNSTREAM);
     PositionWithAffinityTemplate();
     ~PositionWithAffinityTemplate();
 
     EAffinity affinity() const { return m_affinity; }
     const PositionType& position() const { return m_position; }
 
+    // Returns true if both |this| and |other| is null or both |m_position|
+    // and |m_affinity| equal.
+    bool operator==(const PositionWithAffinityTemplate& other) const;
+    bool operator!=(const PositionWithAffinityTemplate& other) const { return !operator==(other); }
+
+    bool isNotNull() const { return m_position.isNotNull(); }
+    bool isNull() const { return m_position.isNull(); }
+
     DEFINE_INLINE_TRACE()
     {
         visitor->trace(m_position);
     }
 
 private:
     PositionType m_position;
     EAffinity m_affinity;
 };
 
 extern template class CORE_EXTERN_TEMPLATE_EXPORT PositionWithAffinityTemplate<Position>;
 
 using PositionWithAffinity = PositionWithAffinityTemplate<Position>;
 
 } // namespace blink
 
 #endif // PositionWithAffinity_h