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

Review URL: https://codereview.chromium.org/1316233002
diff --git a/samplecode/SampleStringArt.cpp b/samplecode/SampleStringArt.cpp
index 5bc667b..74f3bd8 100644
--- a/samplecode/SampleStringArt.cpp
+++ b/samplecode/SampleStringArt.cpp
@@ -59,8 +59,8 @@
 
     SkView::Click* onFindClickHandler(SkScalar x, SkScalar y, unsigned) override {
         fAngle = x/width();
-        this->inval(NULL);
-        return NULL;
+        this->inval(nullptr);
+        return nullptr;
     }
 private: