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

tools/sk_tool_utils.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef sk_tool_utils_DEFINED
 #define sk_tool_utils_DEFINED
 
@@ skipping 26 matching lines @@
     void write_pixels(SkCanvas*, const SkBitmap&, int x, int y, SkColorType, SkAlphaType);
 
     // private to sk_tool_utils
     SkTypeface* create_font(const char* name, SkTypeface::Style );
     SkTypeface* resource_font(const char* name, SkTypeface::Style );
 
     /** Returns a newly created CheckerboardShader. */
     SkShader* create_checkerboard_shader(SkColor c1, SkColor c2, int size);
 
     /** Draw a checkerboard pattern in the current canvas, restricted to
-        the current clip. */
+        the current clip, using SkXfermode::kSrc_Mode. */
     void draw_checkerboard(SkCanvas* canvas,
                            SkColor color1,
                            SkColor color2,
                            int size);
 
     /** A default checkerboard. */
     inline void draw_checkerboard(SkCanvas* canvas) {
         sk_tool_utils::draw_checkerboard(canvas, 0xFF999999, 0xFF666666, 8);
     }
 
 }  // namespace sk_tool_utils
 
 #endif  // sk_tool_utils_DEFINED