Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.cpp b/Source/platform/graphics/gpu/DrawingBuffer.cpp
index 4235403319b5725dfe7b7a930d4e125103d85d38..1478ff8fcd888762da886c401ba65ab1b2823ffc 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -786,7 +786,7 @@ void DrawingBuffer::paintRenderingResultsToCanvas(ImageBuffer* imageBuffer)
 PassRefPtr<Uint8ClampedArray> DrawingBuffer::paintRenderingResultsToImageData(int& width, int& height)
 {
     if (m_attributes.premultipliedAlpha)
-        return 0;
+        return nullptr;
 
     width = size().width();
     height = size().height();
@@ -795,7 +795,7 @@ PassRefPtr<Uint8ClampedArray> DrawingBuffer::paintRenderingResultsToImageData(in
     dataSize *= width;
     dataSize *= height;
     if (dataSize.hasOverflowed())
-        return 0;
+        return nullptr;
 
     RefPtr<Uint8ClampedArray> pixels = Uint8ClampedArray::createUninitialized(width * height * 4);