[Sheriff] Disable system_health.memory_desktop/load:news:bbc:2018 on Mac

TBR=ulan@chromium.org

Bug: 999004
Change-Id: If8eca8c8b94f91e67765f71d25b24eb53204b599
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1782854Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693167}

tools/perf/expectations.config

@@ -336,6 +336,7 @@ crbug.com/959418 [ chromeos ] system_health.memory_desktop/long_running:tools:gm
 crbug.com/959418 [ chromeos ] system_health.memory_desktop/load:games:spychase:2018 [ Skip ]
 crbug.com/959418 [ chromeos ] system_health.memory_desktop/long_running:tools:gmail-foreground [ Skip ]
 crbug.com/999004 [ linux ] system_health.memory_desktop/load:news:bbc:2018 [ Skip ]
+crbug.com/999004 [ mac ] system_health.memory_desktop/load:news:bbc:2018 [ Skip ]
 
 # Benchmark: system_health.memory_mobile
 crbug.com/914390 [ android-nexus-5 ] system_health.memory_mobile/browse:chrome:newtab [ Skip ]