Avoid hang in OncePtr test with --threads 1

src/core/SkTaskGroup.cpp

 /*
  * Copyright 2014 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 "SkRunnable.h"
 #include "SkSemaphore.h"
@@ skipping 178 matching lines @@
     // and that's the only way to decrement fWorkAvailable.
     // So fWorkAvailable may overcount actual the work available.
     // We make do, but this means some worker threads may wake spuriously.
     SkSemaphore fWorkAvailable;
 
     // These are only changed in a single-threaded context.
     SkTDArray<SkThread*> fThreads;
     static ThreadPool* gGlobal;
 
     friend struct SkTaskGroup::Enabler;
+    friend int ::sk_parallel_for_thread_count();
 };
 ThreadPool* ThreadPool::gGlobal = nullptr;
 
 }  // namespace
 
 SkTaskGroup::Enabler::Enabler(int threads) {
     SkASSERT(ThreadPool::gGlobal == nullptr);
     if (threads != 0) {
         ThreadPool::gGlobal = new ThreadPool(threads);
     }
 }
 
 SkTaskGroup::Enabler::~Enabler() { delete ThreadPool::gGlobal; }
 
 SkTaskGroup::SkTaskGroup() : fPending(0) {}
 
 void SkTaskGroup::wait()                            { ThreadPool::Wait(&fPending); }
 void SkTaskGroup::add(SkRunnable* task)             { ThreadPool::Add(task, &fPending); }
 void SkTaskGroup::add(void (*fn)(void*), void* arg) { ThreadPool::Add(fn, arg, &fPending); }
 void SkTaskGroup::batch (void (*fn)(void*), void* args, int N, size_t stride) {
     ThreadPool::Batch(fn, args, N, stride, &fPending);
 }
 
+int sk_parallel_for_thread_count() {
+    if (ThreadPool::gGlobal != nullptr) {
+        return ThreadPool::gGlobal->fThreads.count();
+    }
+    return 0;
+}