sk_tool_utils::draw_checkerboard uses SkXfermode::kSrc_Mode to fix valgrind error.

tools/sk_tool_utils.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 "sk_tool_utils.h"
 #include "sk_tool_utils_flags.h"
 
@@ skipping 55 matching lines @@
     bm.eraseColor(c1);
     bm.eraseArea(SkIRect::MakeLTRB(0, 0, size, size), c2);
     bm.eraseArea(SkIRect::MakeLTRB(size, size, 2 * size, 2 * size), c2);
     return SkShader::CreateBitmapShader(
             bm, SkShader::kRepeat_TileMode, SkShader::kRepeat_TileMode);
 }
 
 void draw_checkerboard(SkCanvas* canvas, SkColor c1, SkColor c2, int size) {
     SkPaint paint;
     paint.setShader(create_checkerboard_shader(c1, c2, size))->unref();
+    paint.setXfermodeMode(SkXfermode::kSrc_Mode);
     canvas->drawPaint(paint);
 }
 
 }  // namespace sk_tool_utils