Use C++11 features in core/html

Source/core/html/HTMLLinkElement.cpp

Index: Source/core/html/HTMLLinkElement.cpp
diff --git a/Source/core/html/HTMLLinkElement.cpp b/Source/core/html/HTMLLinkElement.cpp
index 6851d7ca399ae4609b67af2671fef8af52f2b2ce..6095285cc7510c244dd03e61cbb0e45745021296 100644
--- a/Source/core/html/HTMLLinkElement.cpp
+++ b/Source/core/html/HTMLLinkElement.cpp
@@ -205,7 +205,7 @@ LinkResource* HTMLLinkElement::linkResourceToProcess()
     bool visible = inDocument() && !m_isInShadowTree;
     if (!visible) {
         ASSERT(!linkStyle() || !linkStyle()->hasSheet());
-        return 0;
+        return nullptr;
     }
 
     if (!m_link) {
@@ -227,14 +227,14 @@ LinkResource* HTMLLinkElement::linkResourceToProcess()
 LinkStyle* HTMLLinkElement::linkStyle() const
 {
     if (!m_link || m_link->type() != LinkResource::Style)
-        return 0;
+        return nullptr;
     return static_cast<LinkStyle*>(m_link.get());
 }
 
 LinkImport* HTMLLinkElement::linkImport() const
 {
     if (!m_link || m_link->type() != LinkResource::Import)
-        return 0;
+        return nullptr;
     return static_cast<LinkImport*>(m_link.get());
 }
 
@@ -242,7 +242,7 @@ Document* HTMLLinkElement::import() const
 {
     if (LinkImport* link = linkImport())
         return link->importedDocument();
-    return 0;
+    return nullptr;
 }
 
 void HTMLLinkElement::process()