Use a spinlock in SkOnce.

src/ports/SkAtomics_android.h

Index: src/ports/SkAtomics_android.h
diff --git a/src/ports/SkAtomics_android.h b/src/ports/SkAtomics_android.h
index 5cc8ba54301f7d0b5c740b452eb282056116d3dd..e1b99524bd62a002befdf9487730fca7f952b72f 100644
--- a/src/ports/SkAtomics_android.h
+++ b/src/ports/SkAtomics_android.h
@@ -36,6 +36,13 @@ static inline __attribute__((always_inline)) int32_t sk_atomic_conditional_inc(i
     }
 }
 
+static inline __attribute___((always_inline)) bool sk_atomic_cas(int32_t* addr,
+                                                                 int32_t before,
+                                                                 int32_t after) {
+    // android_atomic_cas returns *addr == before as an int.
+    return android_atomic_cas(before, after, addr) != 0;
+}
+
 static inline __attribute__((always_inline)) void sk_membar_acquire__after_atomic_conditional_inc() {
     //HACK: Android is actually using full memory barriers.
     //      Should this change, uncomment below.