Use C++11 features in core/html

Source/core/html/URLRegistry.h

Index: Source/core/html/URLRegistry.h
diff --git a/Source/core/html/URLRegistry.h b/Source/core/html/URLRegistry.h
index 31a1605c67a0448d2a6f14c01d53baa59704e112..0e655c60dfdf834a21ceb8b5da5b2c513b4ec3ba 100644
--- a/Source/core/html/URLRegistry.h
+++ b/Source/core/html/URLRegistry.h
@@ -53,7 +53,7 @@ public:
     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; }
 };