Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/core/css/resolver/CSSToStyleMap.cpp

Index: Source/core/css/resolver/CSSToStyleMap.cpp
diff --git a/Source/core/css/resolver/CSSToStyleMap.cpp b/Source/core/css/resolver/CSSToStyleMap.cpp
index 406b16c3d5bf155e03b5a1fc63cd26f9a542f4fc..6af0f49f01ae1dcc5c2ee0d3e47a7b7d68771346 100644
--- a/Source/core/css/resolver/CSSToStyleMap.cpp
+++ b/Source/core/css/resolver/CSSToStyleMap.cpp
@@ -481,7 +481,7 @@ PassRefPtr<TimingFunction> CSSToStyleMap::animationTimingFunction(CSSValue* valu
         default:
             break;
         }
-        return 0;
+        return nullptr;
     }
 
     if (value->isCubicBezierTimingFunctionValue()) {
@@ -492,7 +492,7 @@ PassRefPtr<TimingFunction> CSSToStyleMap::animationTimingFunction(CSSValue* valu
         return StepsTimingFunction::create(stepsTimingFunction->numberOfSteps(), stepsTimingFunction->stepAtStart());
     }
 
-    return 0;
+    return nullptr;
 }
 
 void CSSToStyleMap::mapAnimationTimingFunction(CSSAnimationData* animation, CSSValue* value) const