Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/platform/mhtml/MHTMLArchive.cpp

Index: Source/platform/mhtml/MHTMLArchive.cpp
diff --git a/Source/platform/mhtml/MHTMLArchive.cpp b/Source/platform/mhtml/MHTMLArchive.cpp
index 86acbe1b50a2fd702e3d614c41b8e2bc7d5b922c..e22207361fd23399127fff7b861bb3bfe97b3784 100644
--- a/Source/platform/mhtml/MHTMLArchive.cpp
+++ b/Source/platform/mhtml/MHTMLArchive.cpp
@@ -99,12 +99,12 @@ PassRefPtr<MHTMLArchive> MHTMLArchive::create(const KURL& url, SharedBuffer* dat
 {
     // For security reasons we only load MHTML pages from local URLs.
     if (!SchemeRegistry::shouldTreatURLSchemeAsLocal(url.protocol()))
-        return 0;
+        return nullptr;
 
     MHTMLParser parser(data);
     RefPtr<MHTMLArchive> mainArchive = parser.parseArchive();
     if (!mainArchive)
-        return 0; // Invalid MHTML file.
+        return nullptr; // Invalid MHTML file.
 
     // Since MHTML is a flat format, we need to make all frames aware of all resources.
     for (size_t i = 0; i < parser.frameCount(); ++i) {