Disable system_health.common_desktop/long_running:tools:gmail-foreground on mac

NOTRY=True
TBR=charliea@chromium.org

Bug: 903417
Change-Id: I8712e992b8abe19f05a1eb1f71c4908e6ac4369b
Reviewed-on: https://chromium-review.googlesource.com/c/1327160Reviewed-by: Emily Hanley <eyaich@chromium.org>
Commit-Queue: Emily Hanley <eyaich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606546}

tools/perf/expectations.config

@@ -224,6 +224,7 @@ crbug.com/894744 [ Android ] startup.mobile/intent:warm:bbc [ Skip ]
 crbug.com/773084 [ Mac ] system_health.common_desktop/browse:tools:maps [ Skip ]
 crbug.com/839411 [ Win ] system_health.common_desktop/browse:social:twitter_infinite_scroll [ Skip ]
 crbug.com/846022 [ Linux ] system_health.common_desktop/browse:social:twitter_infinite_scroll [ Skip ]
+crbug.com/903417 [ Mac ] system_health.common_desktop/long_running:tools:gmail-foreground [ Skip ]
 
 # Benchmark: system_health.common_mobile
 crbug.com/900909 [ Android ] system_health.common_mobile/browse:chrome:omnibox [ Skip ]