C++11: Replace 0 with nullptr where applicable in layout code.

Source/core/layout/svg/ReferenceFilterBuilder.cpp

Index: Source/core/layout/svg/ReferenceFilterBuilder.cpp
diff --git a/Source/core/layout/svg/ReferenceFilterBuilder.cpp b/Source/core/layout/svg/ReferenceFilterBuilder.cpp
index dc4d3032acb910b6e92bb92c2c72051f936ccd93..754c52693e922788605df649173d264596a84c83 100644
--- a/Source/core/layout/svg/ReferenceFilterBuilder.cpp
+++ b/Source/core/layout/svg/ReferenceFilterBuilder.cpp
@@ -42,12 +42,12 @@
 
 namespace blink {
 
-HashMap<const FilterOperation*, OwnPtr<DocumentResourceReference>>* ReferenceFilterBuilder::documentResourceReferences = 0;
+HashMap<const FilterOperation*, OwnPtr<DocumentResourceReference>>* ReferenceFilterBuilder::documentResourceReferences = nullptr;
 
 DocumentResourceReference* ReferenceFilterBuilder::documentResourceReference(const FilterOperation* filterOperation)
 {
     if (!documentResourceReferences)
-        return 0;
+        return nullptr;
 
     return documentResourceReferences->get(filterOperation);
 }