[Benchmark] Disable a failing story on Nexus 5X

Disable failing story system_health.memory_mobile/load:tools:dropbox:2019
on Nexus 5X.

TBR=wenbinzhang@google.com
NOTRY=true

Bug: 1128019
Change-Id: I6ab6b976b57d644d56db92423b2881a4d27fae1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412980Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807340}

tools/perf/expectations.config

@@ -379,6 +379,7 @@ crbug.com/1097719 [ android-nexus-5x android-webview ] system_health.memory_mobi
 crbug.com/1112337 [ android ] system_health.memory_mobile/browse:news:cricbuzz:2019 [ Skip ]
 crbug.com/1113921 [ android-go ] system_health.memory_mobile/load:tools:dropbox:2019 [ Skip ]
 crbug.com/1128019 [ android-pixel-2 ] system_health.memory_mobile/load:tools:dropbox:2019 [ Skip ]
+crbug.com/1128019 [ android-nexus-5x ] system_health.memory_mobile/load:tools:dropbox:2019 [ Skip ]
 
 # Benchmark: tab_switching.typical_25
 crbug.com/747026 [ mac ] tab_switching.typical_25/multitab:misc:typical24 [ Skip ]