Add cast and assignment operators to SkAtomic.

include/core/SkAtomics.h

Index: include/core/SkAtomics.h
diff --git a/include/core/SkAtomics.h b/include/core/SkAtomics.h
index 7c5294b76da582e56414cb2e7ab842d14e48b8fc..e947d1a9a47775d1673761ab9f29657cb2e757a0 100644
--- a/include/core/SkAtomics.h
+++ b/include/core/SkAtomics.h
@@ -57,6 +57,17 @@ public:
         sk_atomic_store(&fVal, val, mo);
     }
 
+    // Alias for .load(sk_memory_order_seq_cst).
+    operator T() const {
+        return this->load();
+    }
+
+    // Alias for .store(v, sk_memory_order_seq_cst).
+    T operator=(const T& v) {
+        this->store(v);
+        return v;
+    }
+
     T fetch_add(const T& val, sk_memory_order mo = sk_memory_order_seq_cst) {
         return sk_atomic_fetch_add(&fVal, val, mo);
     }