Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/platform/audio/HRTFElevation.cpp

Index: Source/platform/audio/HRTFElevation.cpp
diff --git a/Source/platform/audio/HRTFElevation.cpp b/Source/platform/audio/HRTFElevation.cpp
index 6c8903def617d8d020067bf4bcbe3ef811516a70..049e114c8bed0aa56b58291ce59cfeb01428a55d 100644
--- a/Source/platform/audio/HRTFElevation.cpp
+++ b/Source/platform/audio/HRTFElevation.cpp
@@ -74,7 +74,7 @@ static PassRefPtr<AudioBus> getConcatenatedImpulseResponsesForSubject(const Stri
         RefPtr<AudioBus> concatenatedImpulseResponses(AudioBus::loadPlatformResource(subjectName.utf8().data(), ResponseSampleRate));
         ASSERT(concatenatedImpulseResponses);
         if (!concatenatedImpulseResponses)
-            return 0;
+            return nullptr;
 
         bus = concatenatedImpulseResponses;
         audioBusMap.set(subjectName, bus);
@@ -88,7 +88,7 @@ static PassRefPtr<AudioBus> getConcatenatedImpulseResponsesForSubject(const Stri
     bool isBusGood = responseLength == expectedLength && bus->numberOfChannels() == 2;
     ASSERT(isBusGood);
     if (!isBusGood)
-        return 0;
+        return nullptr;
 
     return bus;
 }