Avoid hang in OncePtr test with --threads 1

Avoid hang in OncePtr test when using "dm --threads 1".

The test will hang the threads until sk_num_cores() threads have run
the code. This requires that sk_num_cores() threads to be run in
parallel, which the global thread pool will not do if the thread count
is smaller than sk_num_cores().

BUG=skia:

Review URL: https://codereview.chromium.org/1419593004

src/core/SkTaskGroup.cpp

@@ -196,6 +196,7 @@ private:
     static ThreadPool* gGlobal;
 
     friend struct SkTaskGroup::Enabler;
+    friend int ::sk_parallel_for_thread_count();
 };
 ThreadPool* ThreadPool::gGlobal = nullptr;
 
@@ -219,3 +220,9 @@ 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;
+}

src/core/SkTaskGroup.h

@@ -53,6 +53,8 @@ private:
 // 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) {

tests/OncePtrTest.cpp

@@ -18,8 +18,10 @@ DEF_TEST(OncePtr, r) {
         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);