Disable a flaky perf test

Disable flaky system_health.memory_desktop/browse:news:hackernews:2018
on desktop perf bots.

TBR=crouleau@chromium.org
NOTRY=true

Bug: 1008028
Change-Id: I15b7f313a9c409c1b7a107c4a1f22f5f057d9570
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1826995


Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700309}