Add subtract to atomics.

include/ports/SkAtomics_atomic.h

Index: include/ports/SkAtomics_atomic.h
diff --git a/include/ports/SkAtomics_atomic.h b/include/ports/SkAtomics_atomic.h
index ddbf7c3f372124be0881cc4c49308fc5b7f50ad3..64ee823f90d290da335da093beb7165ba7cfde28 100644
--- a/include/ports/SkAtomics_atomic.h
+++ b/include/ports/SkAtomics_atomic.h
@@ -32,6 +32,12 @@ T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order mo) {
 }
 
 template <typename T>
+T sk_atomic_fetch_sub(T* ptr, T val, sk_memory_order mo) {
+    // All values of mo are valid.
+    return __atomic_fetch_sub(ptr, val, mo);
+}
+
+template <typename T>
 bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired,
                                 sk_memory_order success,
                                 sk_memory_order failure) {