Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/rendering/RenderWidget.cpp

Index: Source/core/rendering/RenderWidget.cpp
diff --git a/Source/core/rendering/RenderWidget.cpp b/Source/core/rendering/RenderWidget.cpp
index e91106844fca0d875ec688de2e51dd624ec6ce91..6be3a4c2a63c7c1fb0e72d82c18790ee6a1cd8f6 100644
--- a/Source/core/rendering/RenderWidget.cpp
+++ b/Source/core/rendering/RenderWidget.cpp
@@ -86,7 +86,7 @@ static void moveWidgetToParentSoon(Widget* child, FrameView* parent)
 
 RenderWidget::RenderWidget(Element* element)
     : RenderReplaced(element)
-    , m_widget(0)
+    , m_widget(nullptr)
     // Reference counting is used to prevent the widget from being
     // destroyed while inside the Widget code, which might not be
     // able to handle that.
@@ -105,7 +105,7 @@ void RenderWidget::willBeDestroyed()
         cache->remove(this);
     }
 
-    setWidget(0);
+    setWidget(nullptr);
 
     RenderReplaced::willBeDestroyed();
 }
@@ -343,7 +343,7 @@ void RenderWidget::widgetPositionsUpdated()
 
 void RenderWidget::clearWidget()
 {
-    m_widget = 0;
+    m_widget = nullptr;
 }
 
 bool RenderWidget::nodeAtPoint(const HitTestRequest& request, HitTestResult& result, const HitTestLocation& locationInContainer, const LayoutPoint& accumulatedOffset, HitTestAction action)