Avoid hang in OncePtr test with --threads 1

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"
 
 DEF_TEST(OncePtr, r) {
     SkOncePtr<int> once;
 
     static SkAtomic<int> calls(0);
     auto create = [&] {
         calls.fetch_add(1);
         return new int(5);
     };
 
-    SkAtomic<int> force_a_race(sk_num_cores());
-
+    SkAtomic<int> force_a_race(sk_parallel_for_thread_count());
+    if (force_a_race < 1) {
+        return;
+    }
     sk_parallel_for(sk_num_cores()*4, [&](size_t) {
         force_a_race.fetch_add(-1);
         while (force_a_race.load() > 0);
 
         int* n = once.get(create);
         REPORTER_ASSERT(r, *n == 5);
     });
     REPORTER_ASSERT(r, calls.load() == 1);
 }
 
 /* TODO(mtklein): next CL
 
 SK_DECLARE_STATIC_ONCE(once_noptr);
 DEF_TEST(OnceNoPtr, r) {
     static SkAtomic<int> calls(0);
 
     SkAtomic<int> force_a_race(sk_num_cores());
     sk_parallel_for(sk_num_cores()*4, [&](size_t) {
         force_a_race.fetch_add(-1);
         while (force_a_race.load() > 0);
 
         SkOnce(&once_noptr, [&] { calls.fetch_add(1); });
     });
     REPORTER_ASSERT(r, calls.load() == 1);
 }
 */