Modernize atomics in SkTaskGroup's threadpool.

src/core/SkTaskGroup.h

Index: src/core/SkTaskGroup.h
diff --git a/src/core/SkTaskGroup.h b/src/core/SkTaskGroup.h
index 14a52c6aac1aadd1d3393466200fec43f4bf0e45..8c7369da25caa9b124bc3486f74ce701c086095e 100644
--- a/src/core/SkTaskGroup.h
+++ b/src/core/SkTaskGroup.h
@@ -9,6 +9,7 @@
 #define SkTaskGroup_DEFINED
 
 #include "SkTypes.h"
+#include "SkAtomics.h"
 
 struct SkRunnable;
 
@@ -45,7 +46,7 @@ private:
     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