Disable media.tough_video_cases on Mac
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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#381695}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk