Factor out checkerboard function in gm and sampleapp into tools.

tools/Checkerboard.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * 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;
+class SkShader;
+
+namespace sk_tools {
+
+/** Returns a newly created CheckerboardShader. */
+SkShader* CreateCheckerboardShader(SkColor c1, SkColor c2, int size);
+
+/** Draw a checkerboard pattern in the current canvas, restricted to
+    the current clip. */
+void DrawCheckerboard(SkCanvas* canvas,
+                      SkColor color1,
+                      SkColor color2,
+                      int size);
+
+/** A default checkerboard. */
+inline void DrawCheckerboard(SkCanvas* canvas) {
+    sk_tools::DrawCheckerboard(canvas, 0xFF999999, 0xFF666666, 8);
+}
+
+}  // namespace sk_tools
+
+#endif  // Checkerboard_DEFINED