Use C++11 features in core/html

Source/core/html/imports/LinkImport.cpp

 /*
  * 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 47 matching lines @@
     if (m_child) {
         m_child->clearClient();
         m_child = nullptr;
     }
 #endif
 }
 
 Document* LinkImport::importedDocument() const
 {
     if (!m_child || !m_owner || !m_owner->inDocument())
-        return 0;
+        return nullptr;
     if (m_child->loader()->hasError())
-        return 0;
+        return nullptr;
     return m_child->document();
 }
 
 void LinkImport::process()
 {
     if (m_child)
         return;
     if (!m_owner)
         return;
     if (!shouldLoadResource())
@@ skipping 60 matching lines @@
 }
 
 void LinkImport::trace(Visitor* visitor)
 {
     visitor->trace(m_child);
     HTMLImportChildClient::trace(visitor);
     LinkResource::trace(visitor);
 }
 
 } // namespace blink