Factor out checkerboard function in gm and sampleapp into tools.

tools/Checkerboard.cpp

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "Checkerboard.h"
+#include "SkCanvas.h"
+#include "SkShader.h"
+
+SkShader* sk_tools::CreateCheckerboardShader(
+        SkColor c1, SkColor c2, int size) {
+    SkBitmap bm;
+    bm.allocN32Pixels(2 * size, 2 * size);
+    bm.eraseColor(c1);
+    bm.eraseArea(SkIRect::MakeLTRB(0, 0, size, size), c2);
+    bm.eraseArea(SkIRect::MakeLTRB(size, size, 2 * size, 2 * size), c2);
+    return SkShader::CreateBitmapShader(
+            bm, SkShader::kRepeat_TileMode, SkShader::kRepeat_TileMode);
+}
+
+void sk_tools::DrawCheckerboard(SkCanvas* canvas,
+                                    SkColor c1, SkColor c2, int size) {
+    SkPaint paint;
+    paint.setShader(CreateCheckerboardShader(c1, c2, size))->unref();
+    canvas->drawPaint(paint);
+}