Add subtract to atomics.

include/ports/SkAtomics_std.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkAtomics_std_DEFINED
 #define SkAtomics_std_DEFINED
 
@@ skipping 21 matching lines @@
 }
 
 template <typename T>
 T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order mo) {
     // All values of mo are valid.
     std::atomic<T>* ap = reinterpret_cast<std::atomic<T>*>(ptr);
     return std::atomic_fetch_add_explicit(ap, val, (std::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.
+    std::atomic<T>* ap = reinterpret_cast<std::atomic<T>*>(ptr);
+    return std::atomic_fetch_sub_explicit(ap, val, (std::memory_order)mo);
+}
+
+template <typename T>
 bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired,
                                 sk_memory_order success,
                                 sk_memory_order failure) {
     // All values of success are valid.
     SkASSERT(failure == sk_memory_order_relaxed ||
              failure == sk_memory_order_seq_cst ||
              failure == sk_memory_order_acquire ||
              failure == sk_memory_order_consume);
     SkASSERT(failure <= success);
     std::atomic<T>* ap = reinterpret_cast<std::atomic<T>*>(ptr);
     return std::atomic_compare_exchange_strong_explicit(ap, expected, desired,
                                                         (std::memory_order)success,
                                                         (std::memory_order)failure);
 }
 
 template <typename T>
 T sk_atomic_exchange(T* ptr, T val, sk_memory_order mo) {
     // All values of mo are valid.
     std::atomic<T>* ap = reinterpret_cast<std::atomic<T>*>(ptr);
     return std::atomic_exchange_explicit(ap, val, (std::memory_order)mo);
 }
 
 #endif//SkAtomics_std_DEFINED