Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/plugins/DOMMimeType.cpp

Index: Source/core/plugins/DOMMimeType.cpp
diff --git a/Source/core/plugins/DOMMimeType.cpp b/Source/core/plugins/DOMMimeType.cpp
index 84b08e7bac2bd2e91167aa57d5510624be4cd086..dd315da2e882f916b0e4051cad4b1b6f5a98bf8b 100644
--- a/Source/core/plugins/DOMMimeType.cpp
+++ b/Source/core/plugins/DOMMimeType.cpp
@@ -70,7 +70,7 @@ PassRefPtrWillBeRawPtr<DOMPlugin> DOMMimeType::enabledPlugin() const
     // to bounce through the page or mainframe or loader to get there.
     // Something like: m_frame->host()->client()->allowPlugins().
     if (!m_frame || !m_frame->page() || !m_frame->page()->mainFrame()->loader().allowPlugins(NotAboutToInstantiatePlugin))
-        return 0;
+        return nullptr;
 
     return DOMPlugin::create(m_pluginData.get(), m_frame, m_pluginData->mimePluginIndices()[m_index]);
 }