nullptr -> "sans-serif" fixes bold in gm/colorwheel

gm/colorwheel.cpp

Index: gm/colorwheel.cpp
diff --git a/gm/colorwheel.cpp b/gm/colorwheel.cpp
index 530e0c2eed22b3169c30832381806a5859d0dbee..2ebf330d9abf412a818c076dbaec7a769d258346 100644
--- a/gm/colorwheel.cpp
+++ b/gm/colorwheel.cpp
@@ -40,7 +40,7 @@ DEF_SIMPLE_GM(colorwheel, canvas, 256, 256) {
 
 DEF_SIMPLE_GM(colorwheelnative, canvas, 128, 28) {
     SkPaint paint;
-    sk_tool_utils::set_portable_typeface(&paint, nullptr, SkTypeface::kBold);
+    sk_tool_utils::set_portable_typeface(&paint, "sans-serif", SkTypeface::kBold);
     paint.setTextSize(18.0f);
 
     canvas->clear(sk_tool_utils::color_to_565(SK_ColorLTGRAY));