SkAtomics: add asserts for valid memory orders.

include/ports/SkAtomics_atomic.h

 #ifndef SkAtomics_atomic_DEFINED
 #define SkAtomics_atomic_DEFINED
 
 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);
     return __atomic_load_n(ptr, 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);
     __atomic_store_n(ptr, val, mo);
 }
 
 template <typename T>
 T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order mo) {
+    // All values of mo are valid.
     return __atomic_fetch_add(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) {
+    // 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);
     return __atomic_compare_exchange_n(ptr, expected, desired, false/*weak?*/, success, failure);
 }
 
 #endif//SkAtomics_atomic_DEFINED