Add cast and assignment operators to SkAtomic.

tests/AtomicTest.cpp

Index: tests/AtomicTest.cpp
diff --git a/tests/AtomicTest.cpp b/tests/AtomicTest.cpp
index 7eb7a7103b78ccbfeb8859328fc4aad6ab4cf0e4..e9db3f093dd2271155887909113b8a8a60a4ee47 100644
--- a/tests/AtomicTest.cpp
+++ b/tests/AtomicTest.cpp
@@ -54,4 +54,13 @@ DEF_TEST(Atomic, reporter) {
     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);
+    }
 }