Factor out checkerboard function in gm and sampleapp into tools.

gm/offsetimagefilter.cpp

Index: gm/offsetimagefilter.cpp
diff --git a/gm/offsetimagefilter.cpp b/gm/offsetimagefilter.cpp
index 7fcbe0a04db973ec8355bdac280bef6fc92ab0e9..6789df535b53f4175f6c953f28686d008dfaa7c0 100644
--- a/gm/offsetimagefilter.cpp
+++ b/gm/offsetimagefilter.cpp
@@ -5,6 +5,7 @@
  * found in the LICENSE file.
  */
 
+#include "checkerboard.h"
 #include "gm.h"
 #include "SkOffsetImageFilter.h"
 #include "SkBitmapSource.h"
@@ -39,27 +40,6 @@ protected:
         canvas.drawText(str, strlen(str), SkIntToScalar(15), SkIntToScalar(65), paint);
     }
 
-    void make_checkerboard() {
-        fCheckerboard.allocN32Pixels(80, 80);
-        SkCanvas canvas(fCheckerboard);
-        canvas.clear(0x00000000);
-        SkPaint darkPaint;
-        darkPaint.setColor(0xFF404040);
-        SkPaint lightPaint;
-        lightPaint.setColor(0xFFA0A0A0);
-        for (int y = 0; y < 80; y += 16) {
-          for (int x = 0; x < 80; x += 16) {
-            canvas.save();
-            canvas.translate(SkIntToScalar(x), SkIntToScalar(y));
-            canvas.drawRect(SkRect::MakeXYWH(0, 0, 8, 8), darkPaint);
-            canvas.drawRect(SkRect::MakeXYWH(8, 0, 8, 8), lightPaint);
-            canvas.drawRect(SkRect::MakeXYWH(0, 8, 8, 8), lightPaint);
-            canvas.drawRect(SkRect::MakeXYWH(8, 8, 8, 8), darkPaint);
-            canvas.restore();
-          }
-        }
-    }
-
     virtual SkISize onISize() {
         return SkISize::Make(WIDTH, HEIGHT);
     }
@@ -89,7 +69,11 @@ protected:
     virtual void onDraw(SkCanvas* canvas) {
         if (!fInitialized) {
             make_bitmap();
-            make_checkerboard();
+
+            fCheckerboard.allocN32Pixels(80, 80);
+            SkCanvas checkerboardCanvas(fCheckerboard);
+            skiagm::Checkerboard(&checkerboardCanvas, 0xFFA0A0A0, 0xFF404040);
+
             fInitialized = true;
         }
         canvas->clear(0x00000000);