SkAtomics: add asserts for valid memory orders.

include/ports/SkAtomics_std.h

 #ifndef SkAtomics_std_DEFINED
 #define SkAtomics_std_DEFINED
 
 // We try not to depend on the C++ standard library,
 // but these uses of <atomic> should all inline, so we don't feel to bad here.
 #include <atomic>
 
 template <typename T>
 T sk_atomic_load(const T* ptr, sk_memory_order mo) {
+    SkASSERT(mo == sk_memory_order_relaxed ||
+             mo == sk_memory_order_seq_cst ||
+             mo == sk_memory_order_acquire ||
+             mo == sk_memory_order_consume);
     const std::atomic<T>* ap = reinterpret_cast<const std::atomic<T>*>(ptr);
     return std::atomic_load_explicit(ap, (std::memory_order)mo);
 }
 
 template <typename T>
 void sk_atomic_store(T* ptr, T val, sk_memory_order mo) {
+    SkASSERT(mo == sk_memory_order_relaxed ||
+             mo == sk_memory_order_seq_cst ||
+             mo == sk_memory_order_release);
     std::atomic<T>* ap = reinterpret_cast<std::atomic<T>*>(ptr);
     return std::atomic_store_explicit(ap, val, (std::memory_order)mo);
 }
 
 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>
 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);
 }
 
 #endif//SkAtomics_std_DEFINED