Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/page/DOMSelection.cpp

Index: Source/core/page/DOMSelection.cpp
diff --git a/Source/core/page/DOMSelection.cpp b/Source/core/page/DOMSelection.cpp
index 1a9c0ff131816ddd3e1b49689cb569d0fd3a91d6..ac33d7b8d0c4b6a87861b190ec99884659106f80 100644
--- a/Source/core/page/DOMSelection.cpp
+++ b/Source/core/page/DOMSelection.cpp
@@ -352,11 +352,11 @@ void DOMSelection::extend(Node* node, int offset, ExceptionState& exceptionState
 PassRefPtr<Range> DOMSelection::getRangeAt(int index, ExceptionState& exceptionState)
 {
     if (!m_frame)
-        return 0;
+        return nullptr;
 
     if (index < 0 || index >= rangeCount()) {
         exceptionState.throwDOMException(IndexSizeError, String::number(index) + " is not a valid index.");
-        return 0;
+        return nullptr;
     }
 
     // If you're hitting this, you've added broken multi-range selection support