[OSX][Flaky] Disable an SslCastSocketTest

The test that used to be disabled on Win only is now disabled on Mac as
well. Recent flake:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.12%20Tests/14961

Flakiness dashboard confirming the flakiness:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#showAllRuns=true&testType=components_unittests&tests=SslCastSocketTest.TestConnectEndToEndWithRealSSL

TBR=wjmaclean@chromium.org

Bug: 874491
Change-Id: Ida63c17fc4618d1944f947c3fcf0a0247bc04d57
Reviewed-on: https://chromium-review.googlesource.com/1179882Reviewed-by: Friedrich Horschig <fhorschig@chromium.org>
Commit-Queue: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584068}

components/cast_channel/cast_socket_unittest.cc

@@ -978,8 +978,8 @@ TEST_F(MockCastSocketTest, TestOpenChannelClosedSocket) {
                                   base::Unretained(&handler_)));
 }
 
-// https://crbug.com/874491, flaky on Win
-#if defined(OS_WIN)
+// https://crbug.com/874491, flaky on Win and Mac
+#if defined(OS_WIN) || defined(OS_MACOSX)
 #define MAYBE_TestConnectEndToEndWithRealSSL \
   DISABLED_TestConnectEndToEndWithRealSSL
 #else