fix race to set the new gen ID

include/core/SkAtomics.h

 #ifndef SkAtomics_DEFINED
 #define SkAtomics_DEFINED
 
 // This file is not part of the public Skia API.
 #include "SkTypes.h"
 
 enum sk_memory_order {
     sk_memory_order_relaxed,
     sk_memory_order_consume,
     sk_memory_order_acquire,
@@ skipping 24 matching lines @@
     SkAtomic() {}
 
     // It is essential we return by value rather than by const&.  fVal may change at any time.
     T load(sk_memory_order mo = sk_memory_order_seq_cst) const {
         return sk_atomic_load(&fVal, mo);
     }
 
     void store(const T& val, sk_memory_order mo = sk_memory_order_seq_cst) {
         sk_atomic_store(&fVal, val, mo);
     }
+
+    bool compare_exchange(T* expected, const T& desired,
+                          sk_memory_order success = sk_memory_order_seq_cst,
+                          sk_memory_order failure = sk_memory_order_seq_cst) {
+        return sk_atomic_compare_exchange(&fVal, expected, desired, success, failure);
+    }
 private:
     T fVal;
 };
 
 #if defined(_MSC_VER)
     #include "../ports/SkAtomics_std.h"
 #elif !defined(SK_BUILD_FOR_IOS) && defined(__ATOMIC_RELAXED)
     #include "../ports/SkAtomics_atomic.h"
 #else
     #include "../ports/SkAtomics_sync.h"
@@ skipping 31 matching lines @@
 template <typename T>
 T sk_acquire_load(T* ptr) { return sk_atomic_load(ptr, sk_memory_order_acquire); }
 
 template <typename T>
 void sk_release_store(T* ptr, T val) { sk_atomic_store(ptr, val, sk_memory_order_release); }
 
 inline void sk_membar_acquire__after_atomic_dec() {}
 inline void sk_membar_acquire__after_atomic_conditional_inc() {}
 
 #endif//SkAtomics_DEFINED