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

gm/colorwheel.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Resources.h"
 #include "SkData.h"
 #include "SkImage.h"
@@ skipping 22 matching lines @@
 DEF_SIMPLE_GM(colorwheel, canvas, 256, 256) {
     sk_tool_utils::draw_checkerboard(canvas);
     draw_image(canvas, "color_wheel.png", 0, 0);  // top left
     draw_image(canvas, "color_wheel.gif", 128, 0);  // top right
     draw_image(canvas, "color_wheel.webp", 0, 128);  // bottom left
     draw_image(canvas, "color_wheel.jpg", 128, 128);  // bottom right
 }
 
 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));
     paint.setColor(SK_ColorRED);
     canvas->drawText("R", 1, 8.0f, 20.0f, paint);
     paint.setColor(SK_ColorGREEN);
     canvas->drawText("G", 1, 24.0f, 20.0f, paint);
     paint.setColor(SK_ColorBLUE);
     canvas->drawText("B", 1, 40.0f, 20.0f, paint);
     paint.setColor(SK_ColorCYAN);
     canvas->drawText("C", 1, 56.0f, 20.0f, paint);
     paint.setColor(SK_ColorMAGENTA);
     canvas->drawText("M", 1, 72.0f, 20.0f, paint);
     paint.setColor(SK_ColorYELLOW);
     canvas->drawText("Y", 1, 88.0f, 20.0f, paint);
     paint.setColor(SK_ColorBLACK);
     canvas->drawText("K", 1, 104.0f, 20.0f, paint);
 }