Add subtract to atomics.

include/core/SkAtomics.h

Index: include/core/SkAtomics.h
diff --git a/include/core/SkAtomics.h b/include/core/SkAtomics.h
index d31d9c64fb40ce13a20cc7db9a3e485bae0576d0..7c5294b76da582e56414cb2e7ab842d14e48b8fc 100644
--- a/include/core/SkAtomics.h
+++ b/include/core/SkAtomics.h
@@ -30,6 +30,9 @@ template <typename T>
 T sk_atomic_fetch_add(T*, T, sk_memory_order = sk_memory_order_seq_cst);
 
 template <typename T>
+T sk_atomic_fetch_sub(T*, T, sk_memory_order = sk_memory_order_seq_cst);
+
+template <typename T>
 bool sk_atomic_compare_exchange(T*, T* expected, T desired,
                                 sk_memory_order success = sk_memory_order_seq_cst,
                                 sk_memory_order failure = sk_memory_order_seq_cst);
@@ -58,6 +61,10 @@ public:
         return sk_atomic_fetch_add(&fVal, val, mo);
     }
 
+    T fetch_sub(const T& val, sk_memory_order mo = sk_memory_order_seq_cst) {
+        return sk_atomic_fetch_sub(&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) {