Modernize atomics in SkTaskGroup's threadpool.

include/core/SkAtomics.h

Index: include/core/SkAtomics.h
diff --git a/include/core/SkAtomics.h b/include/core/SkAtomics.h
index 9866db4860cea31df1289391748e23bb7288cc58..be94164227b789c43899cabd38c94d6fe8e3e050 100644
--- a/include/core/SkAtomics.h
+++ b/include/core/SkAtomics.h
@@ -43,6 +43,7 @@ template <typename T>
 class SkAtomic : SkNoncopyable {
 public:
     SkAtomic() {}
+    explicit SkAtomic(const T& val) : fVal(val) {}
 
     // It is essential we return by value rather than by const&.  fVal may change at any time.
     T load(sk_memory_order mo = sk_memory_order_seq_cst) const {
@@ -53,6 +54,10 @@ public:
         sk_atomic_store(&fVal, val, mo);
     }
 
+    T fetch_add(const T& val, sk_memory_order mo = sk_memory_order_seq_cst) {
+        return sk_atomic_fetch_add(&fVal, val, mo);
+    }
+
     bool compare_exchange(T* expected, const T& desired,
                           sk_memory_order success = sk_memory_order_seq_cst,
                           sk_memory_order failure = sk_memory_order_seq_cst) {