Avoid hang in OncePtr test with --threads 1

src/core/SkTaskGroup.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTaskGroup_DEFINED
 #define SkTaskGroup_DEFINED
 
@@ skipping 35 matching lines @@
 
     void add  (void_fn, void* arg);
     void batch(void_fn, void* args, int N, size_t stride);
 
     SkAtomic<int32_t> fPending;
 };
 
 // Returns best estimate of number of CPU cores available to use.
 int sk_num_cores();
 
+int sk_parallel_for_thread_count();
+
 // Call f(i) for i in [0, end).
 template <typename Func>
 void sk_parallel_for(int end, const Func& f) {
     if (end <= 0) { return; }
 
     struct Chunk {
         const Func* f;
         int start, end;
     };
 
@@ skipping 21 matching lines @@
 
     void(*run_chunk)(Chunk*) = [](Chunk* c) {
         for (int i = c->start; i < c->end; i++) {
             (*c->f)(i);
         }
     };
     SkTaskGroup().batch(run_chunk, chunks.get(), nchunks);
 }
 
 #endif//SkTaskGroup_DEFINED