SkAtomics: add asserts for valid memory orders.

include/ports/SkAtomics_std.h

Index: include/ports/SkAtomics_std.h
diff --git a/include/ports/SkAtomics_std.h b/include/ports/SkAtomics_std.h
index f1fb0e2053083cca8953f442ecc50ed469f52374..e89d74981c449c228899419591c717fb3077155f 100644
--- a/include/ports/SkAtomics_std.h
+++ b/include/ports/SkAtomics_std.h
@@ -7,18 +7,26 @@
 
 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);
 }
@@ -27,6 +35,12 @@ 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,