SkOnce: add option to call another cleanup function once at exit.

tests/OnceTest.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkOnce.h"
 #include "SkThreadPool.h"
 #include "Test.h"
@@ skipping 59 matching lines @@
     // Let them race.
     SkThreadPool pool(kThreads);
     for (int i = 0; i < kTasks; i++) {
         pool.add(&racers[i]);
     }
     pool.wait();
 
     // Only one should have done the +=.
     REPORTER_ASSERT(r, 6 == x);
 }
+
+// Test that the atExit option works.
+static int gToDecrement = 1;
+static void noop(int) {}
+static void decrement() { gToDecrement--; }
+static void checkDecremented() { SkASSERT(gToDecrement == 0); }
+
+DEF_TEST(SkOnce_atExit, r) {
+    atexit(checkDecremented);
+    SK_DECLARE_STATIC_ONCE(once);
+    SkOnce(&once, noop, 0, decrement);
+}