Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/accessibility/AXNodeObject.cpp

Index: Source/core/accessibility/AXNodeObject.cpp
diff --git a/Source/core/accessibility/AXNodeObject.cpp b/Source/core/accessibility/AXNodeObject.cpp
index f57327e79df0ee9daea9453a561c079b927fddb5..e0180ff7bda0d4cecd9706862467a7497dffeffa 100644
--- a/Source/core/accessibility/AXNodeObject.cpp
+++ b/Source/core/accessibility/AXNodeObject.cpp
@@ -1509,7 +1509,7 @@ void AXNodeObject::setFocused(bool on)
 
     Document* document = this->document();
     if (!on) {
-        document->setFocusedElement(0);
+        document->setFocusedElement(nullptr);
     } else {
         Node* node = this->node();
         if (node && node->isElementNode()) {
@@ -1517,11 +1517,11 @@ void AXNodeObject::setFocused(bool on)
             // That is a problem when focus is removed from the webpage to chrome, and then returns.
             // In these cases, we need to do what keyboard and mouse focus do, which is reset focus first.
             if (document->focusedElement() == node)
-                document->setFocusedElement(0);
+                document->setFocusedElement(nullptr);
 
             toElement(node)->focus();
         } else {
-            document->setFocusedElement(0);
+            document->setFocusedElement(nullptr);
         }
     }
 }