Reland of SkOncePtr -> SkOnce

tests/OncePtrTest.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 "Test.h"
-#include "SkOncePtr.h"
-#include "SkTaskGroup.h"
-
-SK_DECLARE_STATIC_ONCE_PTR(int, once);
-
-DEF_TEST(OncePtr, r) {
-    static SkAtomic<int> calls(0);
-    auto create = [&] {
-        calls.fetch_add(1);
-        return new int(5);
-    };
-
-    SkTaskGroup().batch(sk_num_cores()*4, [&](size_t) {
-        int* n = once.get(create);
-        REPORTER_ASSERT(r, *n == 5);
-    });
-    REPORTER_ASSERT(r, calls.load() == 1);
-}