Factor out checkerboard function in gm and sampleapp into tools.

gm/checkerboard.h

+/*
+ * Copyright 2015 Google Inc.

[djsollen, 2015/01/07 16:24:20]

move to tools and make the checkerboard changes in a separate CL.

+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef checkerboard_DEFINED
+#define checkerboard_DEFINED
+
+#include "SkColor.h"
+
+class SkCanvas;
+
+namespace skiagm {
+
+/**
+   Draw a checkerboard pattern in the current canvas, restricted to
+   the current clip.
+ */
+void Checkerboard(SkCanvas* canvas, SkColor color1, SkColor color2);
+
+inline void Checkerboard(SkCanvas* canvas) {
+    Checkerboard(canvas, 0xFF999999, 0xFF666666);
+}
+
+}  // namespace skiagm
+
+#endif  // checkerboard_DEFINED