Use C++11 features in core/html

Source/core/html/HTMLLabelElement.cpp

Index: Source/core/html/HTMLLabelElement.cpp
diff --git a/Source/core/html/HTMLLabelElement.cpp b/Source/core/html/HTMLLabelElement.cpp
index 90b73c07f1b85c147111fb9c83660b16b2201038..04255f4ca9724d69b05aea06fc57f269ae7d39d9 100644
--- a/Source/core/html/HTMLLabelElement.cpp
+++ b/Source/core/html/HTMLLabelElement.cpp
@@ -68,7 +68,7 @@ LabelableElement* HTMLLabelElement::control() const
             if (element.supportLabels())
                 return &element;
         }
-        return 0;
+        return nullptr;
     }
 
     if (Element* element = treeScope().getElementById(controlId)) {
@@ -76,7 +76,7 @@ LabelableElement* HTMLLabelElement::control() const
             return toLabelableElement(element);
     }
 
-    return 0;
+    return nullptr;
 }
 
 HTMLFormElement* HTMLLabelElement::formOwner() const