Add negative test case for CompareAndSwap.

base/atomicops_unittest.cc

Index: base/atomicops_unittest.cc
diff --git a/base/atomicops_unittest.cc b/base/atomicops_unittest.cc
index 3fd559747226dab196d7f0cd68fc26c4dd9b3e7d..7298609270f883cf8e4245f67fd623dd90efa8a4 100644
--- a/base/atomicops_unittest.cc
+++ b/base/atomicops_unittest.cc
@@ -89,6 +89,13 @@ static void TestCompareAndSwap() {
   EXPECT_EQ(1, value);
   EXPECT_EQ(0, prev);
 
+  // Verify that CAS will *not* change "value" if it doesn't match the
+  // expected  number. CAS will always return the actual value of the
+  // variable from before any change.
+  AtomicType fail = base::subtle::NoBarrier_CompareAndSwap(&value, 0, 2);
+  EXPECT_EQ(1, value);
+  EXPECT_EQ(1, fail);
+
   // Use test value that has non-zero bits in both halves, more for testing
   // 64-bit implementation on 32-bit platforms.
   const AtomicType k_test_val = (static_cast<uint64_t>(1) <<