Factor out checkerboard function in gm and sampleapp into tools.

gm/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 "SkCanvas.h"
+#include "SkShader.h"
+#include "checkerboard.h"
+
+void skiagm::Checkerboard(SkCanvas* canvas, SkColor c1, SkColor c2) {
+    SkBitmap bm;
+    bm.allocN32Pixels(16, 16);
+    bm.eraseColor(c1);
+    bm.eraseArea(SkIRect::MakeLTRB(0, 0, 8, 8), c2);
+    bm.eraseArea(SkIRect::MakeLTRB(8, 8, 16, 16), c2);
+    SkAutoTUnref<SkShader> shader(SkShader::CreateBitmapShader(
+            bm, SkShader::kRepeat_TileMode, SkShader::kRepeat_TileMode));
+    SkPaint paint;
+    paint.setShader(shader);
+    canvas->drawPaint(paint);
+}