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

tools/sk_tool_utils.cpp

Index: tools/sk_tool_utils.cpp
diff --git a/tools/sk_tool_utils.cpp b/tools/sk_tool_utils.cpp
index 560384fb7cbbcd7e31bac198f52722fe8afb15e2..c3e574a08e3f63b7adede0ce1f6cf4e3112c9179 100644
--- a/tools/sk_tool_utils.cpp
+++ b/tools/sk_tool_utils.cpp
@@ -73,6 +73,7 @@ SkShader* create_checkerboard_shader(SkColor c1, SkColor c2, int size) {
 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);
 }