Factor out checkerboard function in gm and sampleapp into tools.

samplecode/SampleUnpremul.cpp

Index: samplecode/SampleUnpremul.cpp
diff --git a/samplecode/SampleUnpremul.cpp b/samplecode/SampleUnpremul.cpp
index 9591fec159797929a85de6a448defd9f0a205b63..889d06780e2a46bddcc5af247cab93205c530ead 100644
--- a/samplecode/SampleUnpremul.cpp
+++ b/samplecode/SampleUnpremul.cpp
@@ -7,6 +7,7 @@
 
 #include "gm.h"
 
+#include "Checkerboard.h"
 #include "Resources.h"
 #include "SampleCode.h"
 #include "SkBlurMask.h"
@@ -25,9 +26,6 @@
 
 __SK_FORCE_IMAGE_DECODER_LINKING;
 
-// Defined in SampleColorFilter.cpp
-extern SkShader* createChecker();
-
 /**
  *  Interprets c as an unpremultiplied color, and returns the
  *  premultiplied equivalent.
@@ -78,10 +76,7 @@ protected:
     }
 
     void onDrawBackground(SkCanvas* canvas) SK_OVERRIDE {
-        SkPaint paint;
-        SkAutoTUnref<SkShader> shader(createChecker());
-        paint.setShader(shader.get());
-        canvas->drawPaint(paint);
+        sk_tools::DrawCheckerboard(canvas, 0xFFCCCCCC, 0xFFFFFFFF, 12);
     }
 
     void onDrawContent(SkCanvas* canvas) SK_OVERRIDE {