Use a spinlock in SkOnce.

src/ports/SkAtomics_win.h

Index: src/ports/SkAtomics_win.h
diff --git a/src/ports/SkAtomics_win.h b/src/ports/SkAtomics_win.h
index 4fb5e17ec042f949f6632f599187bff05e67cf96..f4d6da4e3e28bdd5ae6c140a0a35962747898d7f 100644
--- a/src/ports/SkAtomics_win.h
+++ b/src/ports/SkAtomics_win.h
@@ -53,6 +53,10 @@ static inline int32_t sk_atomic_conditional_inc(int32_t* addr) {
     }
 }
 
+static inline bool sk_atomic_cas(int32_t* addr, int32_t before, int32_t after) {
+    return _InterlockedCompareExchange(reinterpret_cast<long*>(addr), after, before) == before;
+}
+
 static inline void sk_membar_acquire__after_atomic_conditional_inc() { }
 
 #endif