Use C++11 features in core/html

Source/core/html/imports/LinkImport.cpp

Index: Source/core/html/imports/LinkImport.cpp
diff --git a/Source/core/html/imports/LinkImport.cpp b/Source/core/html/imports/LinkImport.cpp
index 98fd40a5b872cc3e85a95352cfecaadd86771686..326cde05f684d1670f4f93d9b11dfdf8978d28c8 100644
--- a/Source/core/html/imports/LinkImport.cpp
+++ b/Source/core/html/imports/LinkImport.cpp
@@ -65,9 +65,9 @@ LinkImport::~LinkImport()
 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();
 }