Use C++11 features in core/html

Source/core/html/imports/HTMLImportChild.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 167 matching lines @@
 {
     // Doesn't check m_client nullity because we allow
     // clearClient() to reenter.
     m_client = nullptr;
 }
 #endif
 
 HTMLLinkElement* HTMLImportChild::link() const
 {
     if (!m_client)
-        return 0;
+        return nullptr;
     return m_client->link();
 }
 
 // Ensuring following invariants against the import tree:
 // - HTMLImportChild::firstImport() is the "first import" of the DFS order of the import tree.
 // - The "first import" manages all the children that is loaded by the document.
 void HTMLImportChild::normalize()
 {
     if (!loader()->isFirstImport(this) && this->precedes(loader()->firstImport())) {
         HTMLImportChild* oldFirst = loader()->firstImport();
@@ skipping 24 matching lines @@
 
 void HTMLImportChild::trace(Visitor* visitor)
 {
     visitor->trace(m_customElementMicrotaskStep);
     visitor->trace(m_loader);
     visitor->trace(m_client);
     HTMLImport::trace(visitor);
 }
 
 } // namespace blink