Modernize atomics in SkTaskGroup's threadpool.

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
 
 #include "SkTypes.h"
+#include "SkAtomics.h"
 
 struct SkRunnable;
 
 class SkTaskGroup : SkNoncopyable {
 public:
     // Create one of these in main() to enable SkTaskGroups globally.
     struct Enabler : SkNoncopyable {
         explicit Enabler(int threads = -1);  // Default is system-reported core count.
         ~Enabler();
     };
@@ skipping 16 matching lines @@
     // Block until all Tasks previously add()ed to this SkTaskGroup have run.
     // You may safely reuse this SkTaskGroup after wait() returns.
     void wait();
 
 private:
     typedef void(*void_fn)(void*);
 
     void add  (void_fn, void* arg);
     void batch(void_fn, void* args, int N, size_t stride);
 
-    /*atomic*/ int32_t fPending;
+    SkAtomic<int32_t> fPending;
 };
 
 #endif//SkTaskGroup_DEFINED