Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/editing/VisiblePosition.cpp

Index: Source/core/editing/VisiblePosition.cpp
diff --git a/Source/core/editing/VisiblePosition.cpp b/Source/core/editing/VisiblePosition.cpp
index 71205fde7a8476cfb3786f992781c5b2a856f976..0f86a90f633814ad600196d58a7bdde9bf935d81 100644
--- a/Source/core/editing/VisiblePosition.cpp
+++ b/Source/core/editing/VisiblePosition.cpp
@@ -723,12 +723,12 @@ void VisiblePosition::showTreeForThis() const
 PassRefPtr<Range> makeRange(const VisiblePosition &start, const VisiblePosition &end)
 {
     if (start.isNull() || end.isNull())
-        return 0;
+        return nullptr;
 
     Position s = start.deepEquivalent().parentAnchoredEquivalent();
     Position e = end.deepEquivalent().parentAnchoredEquivalent();
     if (s.isNull() || e.isNull())
-        return 0;
+        return nullptr;
 
     return Range::create(s.containerNode()->document(), s.containerNode(), s.offsetInContainerNode(), e.containerNode(), e.offsetInContainerNode());
 }