Use C++11 features in core/html

Source/core/html/HTMLShadowElement.cpp

Index: Source/core/html/HTMLShadowElement.cpp
diff --git a/Source/core/html/HTMLShadowElement.cpp b/Source/core/html/HTMLShadowElement.cpp
index 51533d70f23994997ad4ae612614706a899ad5f2..988c33b00697891c3dae635496e349b05783ff01 100644
--- a/Source/core/html/HTMLShadowElement.cpp
+++ b/Source/core/html/HTMLShadowElement.cpp
@@ -55,13 +55,13 @@ ShadowRoot* HTMLShadowElement::olderShadowRoot()
 {
     ShadowRoot* containingRoot = containingShadowRoot();
     if (!containingRoot)
-        return 0;
+        return nullptr;
 
     document().updateDistributionForNodeIfNeeded(this);
 
     ShadowRoot* older = containingRoot->olderShadowRoot();
     if (!older || !older->shouldExposeToBindings() || older->shadowInsertionPointOfYoungerShadowRoot() != this)
-        return 0;
+        return nullptr;
 
     ASSERT(older->shouldExposeToBindings());
     return older;