Use C++11 features in core/html

Source/core/html/URLRegistry.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 35 matching lines @@
 };
 
 class URLRegistry {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     virtual ~URLRegistry() { }
     virtual void registerURL(SecurityOrigin*, const KURL&, URLRegistrable*) = 0;
     virtual void unregisterURL(const KURL&) = 0;
 
     // These are optional APIs
-    virtual URLRegistrable* lookup(const String&) { ASSERT_NOT_REACHED(); return 0; }
+    virtual URLRegistrable* lookup(const String&) { ASSERT_NOT_REACHED(); return nullptr; }
     virtual bool contains(const String&) { ASSERT_NOT_REACHED(); return false; }
 };
 
 } // namespace blink
 
 #endif // URLRegistry_h