Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/editing/InsertListCommand.cpp

Index: Source/core/editing/InsertListCommand.cpp
diff --git a/Source/core/editing/InsertListCommand.cpp b/Source/core/editing/InsertListCommand.cpp
index e5bedb07b955e4d8231463e3e181ddc517a32178..24a076a0966f5438ce601470b77ef1b3cab18917 100644
--- a/Source/core/editing/InsertListCommand.cpp
+++ b/Source/core/editing/InsertListCommand.cpp
@@ -65,7 +65,7 @@ PassRefPtr<HTMLElement> InsertListCommand::mergeWithNeighboringLists(PassRefPtr<
         mergeIdenticalElements(previousList, list);
 
     if (!list)
-        return 0;
+        return nullptr;
 
     Element* nextSibling = ElementTraversal::nextSibling(*list);
     if (!nextSibling || !nextSibling->isHTMLElement())
@@ -333,7 +333,7 @@ PassRefPtr<HTMLElement> InsertListCommand::listifyParagraph(const VisiblePositio
     VisiblePosition end = endOfParagraph(start, CanSkipOverEditingBoundary);
 
     if (start.isNull() || end.isNull())
-        return 0;
+        return nullptr;
 
     // Check for adjoining lists.
     RefPtr<HTMLElement> listItemElement = createListItemElement(document());