Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/css/StylePropertySerializer.cpp

Index: Source/core/css/StylePropertySerializer.cpp
diff --git a/Source/core/css/StylePropertySerializer.cpp b/Source/core/css/StylePropertySerializer.cpp
index e9ba2af369f2e8f565ee2cc4a9aee26a870f2409..65d7e22c09dd231464c3100174a45de40fa96e83 100644
--- a/Source/core/css/StylePropertySerializer.cpp
+++ b/Source/core/css/StylePropertySerializer.cpp
@@ -570,9 +570,11 @@ String StylePropertySerializer::getLayeredShorthandValue(const StylePropertyShor
                     // Color only belongs in the last layer.
                     if (shorthand.properties()[j] == CSSPropertyBackgroundColor) {
                         if (i != numLayers - 1)
-                            value = 0;
-                    } else if (i) // Other singletons only belong in the first layer.
-                        value = 0;
+                            value = nullptr;
+                    } else if (i) {
+                        // Other singletons only belong in the first layer.
+                        value = nullptr;
+                    }
                 }
             }