Use a spinlock in SkOnce.

src/ports/SkAtomics_none.h

Index: src/ports/SkAtomics_none.h
diff --git a/src/ports/SkAtomics_none.h b/src/ports/SkAtomics_none.h
index 345cb09de4664d99bcd26a81ef9a3fd14fe1e59e..d44791c32c162befc683d56b471f58a542b63d5e 100644
--- a/src/ports/SkAtomics_none.h
+++ b/src/ports/SkAtomics_none.h
@@ -38,6 +38,12 @@ static inline int32_t sk_atomic_conditional_inc(int32_t* addr) {
     return value;
 }
 
+static inline bool sk_atomic_cas(int32_t* addr, int32_t before, int32_t after) {
+    if (*addr != before) return false;
+    *addr = after;
+    return true;
+}
+
 static inline void sk_membar_acquire__after_atomic_conditional_inc() { }
 
 #endif