sheriff: disable CastStreamingApiTestWithPixelOutput on Mac

The EndToEnd test in this suite is flaky on Mac.

TBR=miu@chromium.org

Bug: 841387
Change-Id: I73480393c499dbd05a7891a78b24d7d4ba11d326
Reviewed-on: https://chromium-review.googlesource.com/1052428Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557256}

chrome/browser/extensions/api/cast_streaming/cast_streaming_apitest.cc

@@ -17,6 +17,7 @@
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 #include "base/threading/thread_restrictions.h"
+#include "build/build_config.h"
 #include "chrome/browser/extensions/extension_apitest.h"
 #include "chrome/common/chrome_switches.h"
 #include "content/public/common/content_switches.h"
@@ -337,9 +338,10 @@ class CastStreamingApiTestWithPixelOutput : public CastStreamingApiTest {
 // use the API to send it out.  At the same time, this test launches an
 // in-process Cast receiver, listening on a localhost UDP socket, to receive the
 // content and check whether it matches expectations.
-#if defined(NDEBUG)
+#if defined(NDEBUG) && !defined(OS_MACOSX)
 #define MAYBE_EndToEnd EndToEnd
 #else
+// Flaky on Mac: https://crbug.com/841387
 #define MAYBE_EndToEnd DISABLED_EndToEnd  // crbug.com/396413
 #endif
 IN_PROC_BROWSER_TEST_F(CastStreamingApiTestWithPixelOutput, MAYBE_EndToEnd) {