Add cast and assignment operators to SkAtomic.

tests/AtomicTest.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAtomics.h"
 #include "SkThreadUtils.h"
 #include "SkTypes.h"
@@ skipping 36 matching lines @@
     // Now end the threads
     for (size_t i = 0; i < SK_ARRAY_COUNT(gAdds); i++) {
         threads[i]->join();
         delete threads[i];
     }
     REPORTER_ASSERT(reporter, total == base);
     // Ensure that the returned value from sk_atomic_add is correct.
     int32_t valueToModify = 3;
     const int32_t originalValue = valueToModify;
     REPORTER_ASSERT(reporter, originalValue == sk_atomic_add(&valueToModify, 7));
+
+    {
+        SkAtomic<int> v {0};
+        REPORTER_ASSERT(reporter, 0 == v.load());
+        v = 10;
+        REPORTER_ASSERT(reporter, 10 == v.load());
+        int q = v;
+        REPORTER_ASSERT(reporter, 10 == q);
+    }
 }