[Mac] Better handling of nil for testing macros.

testing/gtest_mac_unittest.mm

Index: testing/gtest_mac_unittest.mm
diff --git a/testing/gtest_mac_unittest.mm b/testing/gtest_mac_unittest.mm
index c5bc47fb806afc1e2d9db5d1ccd58c9755d3fb2e..1376ed1eb2a908e8526b039f5699f7179ad62a14 100644
--- a/testing/gtest_mac_unittest.mm
+++ b/testing/gtest_mac_unittest.mm
@@ -45,3 +45,13 @@ TEST(GTestMac, AssertNSNE) {
 
   ASSERT_NSNE(@"a", @"b");
 }
+
+TEST(GTestMac, ExpectNSNil) {
+  base::mac::ScopedNSAutoreleasePool pool;
+
+  EXPECT_NSEQ(nil, nil);
+  EXPECT_NSNE(nil, @"a");
+  EXPECT_NSNE(@"a", nil);
+
+  // TODO(shess): Test that EXPECT_NSNE(nil, nil) fails.
+}