Factor out checkerboard function in gm and sampleapp into tools.

gm/colorwheel.cpp

Index: gm/colorwheel.cpp
diff --git a/gm/colorwheel.cpp b/gm/colorwheel.cpp
index 93af361720d18573a7621542854c474890d2512c..3b22fa50034c6889aee5d4b154fac4083aba905f 100644
--- a/gm/colorwheel.cpp
+++ b/gm/colorwheel.cpp
@@ -8,27 +8,10 @@
 #include "Resources.h"
 #include "SkData.h"
 #include "SkDecodingImageGenerator.h"
+#include "checkerboard.h"
 #include "gm.h"
 #include "sk_tool_utils.h"
 
-static void checkerboard(
-        SkCanvas* canvas, int w, int h, int size, SkColor c1, SkColor c2) {
-    SkAutoCanvasRestore autoCanvasRestore(canvas, true);
-    canvas->clipRect(SkRect::MakeWH(SkIntToScalar(w), SkIntToScalar(h)));
-    canvas->drawColor(c1);
-    SkPaint paint;
-    paint.setColor(c2);
-    SkScalar s = SkIntToScalar(size);
-    for (int y = 0; y < h; y += size) {
-        SkScalar ty = SkIntToScalar(y);
-        bool oddRow = (y % (2 * size)) != 0;
-        for (int x = oddRow ? size : 0; x < w; x += (2 * size)) {
-            SkScalar tx = SkIntToScalar(x);
-            canvas->drawRect(SkRect::MakeXYWH(tx, ty, s, s), paint);
-        }
-    }
-}
-
 static void draw_bitmap(SkCanvas* canvas, const char* resource, int x, int y) {
     SkBitmap bitmap;
     if (GetResourceAsBitmap(resource, &bitmap)) {
@@ -49,8 +32,7 @@ static void draw_bitmap(SkCanvas* canvas, const char* resource, int x, int y) {
   background and compression artifacts.
  */
 DEF_SIMPLE_GM(colorwheel, canvas, 256, 256) {
-    canvas->clear(SK_ColorWHITE);
-    checkerboard(canvas, 256, 556, 8, 0xFF999999, 0xFF666666);
+    skiagm::Checkerboard(canvas);
     draw_bitmap(canvas, "color_wheel.png", 0, 0);  // top left
     draw_bitmap(canvas, "color_wheel.gif", 128, 0);  // top right
     draw_bitmap(canvas, "color_wheel.webp", 0, 128);  // bottom left