Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/platform/graphics/ImageBuffer.cpp

Index: Source/platform/graphics/ImageBuffer.cpp
diff --git a/Source/platform/graphics/ImageBuffer.cpp b/Source/platform/graphics/ImageBuffer.cpp
index 513de73fd829c42a0f27ef21981198d02e197d4d..b3e7191aff1856593b9cd3fe2665cf733b9ad7b3 100644
--- a/Source/platform/graphics/ImageBuffer.cpp
+++ b/Source/platform/graphics/ImageBuffer.cpp
@@ -256,7 +256,7 @@ PassRefPtr<SkColorFilter> ImageBuffer::createColorSpaceFilter(ColorSpace srcColo
 {
     const uint8_t* lut = ColorSpaceUtilities::getConversionLUT(dstColorSpace, srcColorSpace);
     if (!lut)
-        return 0;
+        return nullptr;
 
     return adoptRef(SkTableColorFilter::CreateARGB(0, lut, lut, lut));
 }
@@ -266,7 +266,7 @@ PassRefPtr<Uint8ClampedArray> getImageData(const IntRect& rect, GraphicsContext*
 {
     float area = 4.0f * rect.width() * rect.height();
     if (area > static_cast<float>(std::numeric_limits<int>::max()))
-        return 0;
+        return nullptr;
 
     RefPtr<Uint8ClampedArray> result = Uint8ClampedArray::createUninitialized(rect.width() * rect.height() * 4);