Style Change: NULL->nullptr
DOCS_PREVIEW= https://skia.org/?cl=1316233002

Review URL: https://codereview.chromium.org/1316233002
diff --git a/gm/imagealphathreshold.cpp b/gm/imagealphathreshold.cpp
index a418f0f..13a0ffa 100644
--- a/gm/imagealphathreshold.cpp
+++ b/gm/imagealphathreshold.cpp
@@ -47,7 +47,7 @@
         SkPaint paint;
         paint.setImageFilter(
             SkAlphaThresholdFilter::Create(region, 0.2f, 0.7f))->unref();
-        canvas->saveLayer(NULL, &paint);
+        canvas->saveLayer(nullptr, &paint);
         paint.setAntiAlias(true);
 
         SkPaint rect_paint;