Disable media.tough_video_cases on Mac

The benchmark is causing lots of test failures on the "Mac HDD Perf (5)"
perf bot (see the associated bug).

BUG=595665
TBR=skyostil
CQ_EXTRA_TRYBOTS=tryserver.chromium.perf:android_s5_perf_cq;tryserver.chromium.perf:winx64_10_perf_cq;tryserver.chromium.perf:mac_retina_perf_cq;tryserver.chromium.perf:linux_perf_cq

Review URL: https://codereview.chromium.org/1807183002

Cr-Commit-Position: refs/heads/master@{#381695}

tools/perf/benchmarks/media.py

@@ -39,8 +39,9 @@ class _MSEMeasurement(page_test.PageTest):
 # android: See media.android.tough_video_cases below
 # win8: crbug.com/531618
 # win7: crbug.com/555079
+# mac: crbug.com/595665
 # crbug.com/565180: Only include cases that report time_to_play
-@benchmark.Disabled('android', 'win8', 'win7')
+@benchmark.Disabled('android', 'win8', 'win7', 'mac')
 class Media(perf_benchmark.PerfBenchmark):
   """Obtains media metrics for key user scenarios."""
   test = media.Media