Add subtract to atomics.

BUG=skia:

Review URL: https://codereview.chromium.org/1348113004

include/core/SkAtomics.h

@@ -29,6 +29,9 @@ void sk_atomic_store(T*, T, sk_memory_order = sk_memory_order_seq_cst);
 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,
@@ -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) {

include/ports/SkAtomics_atomic.h

@@ -31,6 +31,12 @@ T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order mo) {
     return __atomic_fetch_add(ptr, val, 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,

include/ports/SkAtomics_std.h

@@ -38,6 +38,13 @@ T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order mo) {
     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,

include/ports/SkAtomics_sync.h

@@ -45,6 +45,11 @@ T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order) {
     return __sync_fetch_and_add(ptr, val);
 }
 
+template <typename T>
+T sk_atomic_fetch_sub(T* ptr, T val, sk_memory_order) {
+    return __sync_fetch_and_sub(ptr, val);
+}
+
 template <typename T>
 bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired, sk_memory_order, sk_memory_order) {
     T prev = __sync_val_compare_and_swap(ptr, *expected, desired);