Disable three system_health.memory_desktop stories on ChromeOS.

This change skips long_running:tools:gmail-background,
long_running:tools:gmail-foreground, and load:games:spychase:2018
on ChromeOS because they are failing.

The whole system_health.memory_desktop suite is not running, and we
hope disabling some flaky or long-running stories may bring it back.

NOTRY=true
TBR=perezju@chromium.org
CC=cbruni@chromium.org,rnephew@chromium.org

Bug: 959418
Change-Id: I8b500a6d8ed2d8ca07fca31d87728aea1c2867d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610084Reviewed-by: Madeleine Barowsky <mbarowsky@chromium.org>
Commit-Queue: Madeleine Barowsky <mbarowsky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659131}

tools/perf/expectations.config

@@ -273,6 +273,9 @@ crbug.com/874803 [ Win_10 ] system_health.memory_desktop/multitab:misc:typical24
 crbug.com/944978 [ Win_7 ] system_health.memory_desktop/multitab:misc:typical24 [ Skip ]
 crbug.com/934270 [ Win ] system_health.memory_desktop/multitab:misc:typical24:2018 [ Skip ]
 crbug.com/942952 [ ChromeOS ] system_health.memory_desktop/browse:news:hackernews:2018 [ Skip ]
+crbug.com/959418 [ ChromeOS ] system_health.memory_desktop/long_running:tools:gmail-background [ Skip ]
+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 ]
 
 # Benchmark: system_health.memory_mobile
 crbug.com/914390 [ Nexus_5 ] system_health.memory_mobile/browse:chrome:newtab [ Skip ]